Hybrid offshore wind power low-frequency sending-out system and starting control method thereof

Abstract

The invention discloses a hybrid offshore wind power low-frequency sending-out system and a starting control method thereof. The hybrid offshore wind power low-frequency sending-out system comprises a large-scale offshore wind power plant, an onshore alternating current collection bus, an onshore power grid side alternating current collection bus and an onshore alternating current power grid. The output end of the large-scale offshore wind power plant is connected with the land alternating-current collection bus through the offshore alternating-current collection bus and the low-frequency submarine cable, the land alternating-current collection bus is connected with the land power grid side alternating-current collection bus through the land converter station, and the land power grid side alternating-current collection bus is connected with the land alternating-current power grid through the land alternating-current starting loop. According to the system and the starting control method thereof, low-cost and high-reliability electric energy transmission can be realized.

Description

Technical Field

[0001] This invention belongs to the field of flexible low-frequency power transmission technology, and relates to a hybrid offshore wind power low-frequency transmission system and its start-up control method. Background Technology

[0002] With the rapid development of renewable energy, offshore wind power, as an important component of clean energy, has become a key area in the global energy transition. However, large-scale offshore wind farms face numerous challenges in power transmission. Traditional high-voltage direct current (HVDC) transmission systems in the offshore environment need to consider submarine cable costs, system stability, and equipment economics. Currently, there are two main approaches to offshore wind power DC transmission based on DRU-MMC (Diode Rectifier Unit - Modular Multilevel Converter): one is to use grid-connected wind turbines, but their characteristics are still under research; the other is to use grid-connected wind turbines, which require additional auxiliary converters to establish voltage, leading to a reduction in the system's stable operating range and negatively impacting overall stability.

[0003] Furthermore, while onshore DC transmission systems mostly employ true bipolar topologies, offshore wind power DC transmission typically uses pseudo-bipolar structures due to the lower economic viability of additional submarine cables. This limits the system's reliability and scalability. Existing solutions often rely on fully controlled components such as MMCs, resulting in high costs. To address these issues, a low-cost, highly stable hybrid system suitable for low-frequency transmission is needed to achieve efficient grid connection of offshore wind power. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a hybrid offshore wind power low-frequency transmission system and its start-up control method, which can achieve low-cost and high-reliability power transmission.

[0005] To achieve the above objectives, this invention discloses a hybrid offshore wind power low-frequency transmission system, comprising a large-scale offshore wind farm, an onshore AC collection bus, an onshore power grid-side AC collection bus, and an onshore AC power grid. The output end of a large-scale offshore wind farm is connected to the onshore AC collection bus via an offshore AC collection bus and a low-frequency submarine cable. The onshore AC collection bus is connected to the onshore power grid-side AC collection bus via an onshore converter station. The onshore power grid-side AC collection bus is connected to the onshore AC power grid via an onshore AC start-up circuit.

[0006] Furthermore, the large-scale offshore wind farm includes several grid-connected wind turbines, each of which is connected to the offshore AC collection bus via an internal AC collection bus and a step-up transformer.

[0007] Furthermore, the onshore converter station includes a diode uncontrolled rectifier unit, a thyristor rectifier unit, an MMC rectifier station, and an MMC inverter station. The MMC rectifier station and the MMC inverter station form a back-to-back frequency converter. The onshore AC collection bus is connected to the onshore grid-side AC collection bus via the diode uncontrolled rectifier unit and the thyristor rectifier unit. The onshore AC collection bus is also connected to the onshore grid-side AC collection bus via the MMC rectifier station, the diode rectifier unit, and the MMC inverter station.

[0008] Furthermore, the onshore AC collection busbar is connected to the diode uncontrolled rectifier unit via the first switch and the line-side transformer of the first converter station.

[0009] Furthermore, the thyristor rectifier unit is connected to the AC collection busbar on the onshore power grid side via the first converter station grid-side transformer and the second switch.

[0010] Furthermore, the onshore AC collection busbar is connected to the MMC rectifier station via the third switch and the line-side transformer of the second converter station.

[0011] Furthermore, the MMC inverter station is connected to the AC collection busbar on the onshore power grid side via the second converter station's grid-side transformer and the fourth switch.

[0012] Furthermore, the onshore AC starting circuit includes a starting circuit resistor series current measuring device, a starting circuit resistor bypass switch, and a starting resistor. The onshore power grid side AC collecting bus is connected to the onshore AC power grid via the starting circuit resistor series current measuring device, the starting resistor, and the incoming circuit breaker. One end of the starting circuit resistor bypass switch is connected to the line between the starting circuit resistor series current measuring device and the onshore power grid side AC collecting bus, and the other end of the starting circuit resistor bypass switch is connected to the line between the starting resistor and the incoming circuit breaker.

[0013] Furthermore, both the diode uncontrolled rectifier unit and the thyristor rectifier unit are six-pulse.

[0014] This invention discloses a startup control method for a hybrid offshore wind power low-frequency transmission system, comprising: 1) Switch the onshore converter station to the operating state, close the fourth switch, start the MMC inverter station, and use the charging resistor in the onshore converter station to charge the DC bus of the back-to-back frequency converter using the onshore AC power grid. 2) Start the MMC rectifier station, close the third switch, and after the DC voltage stabilizes, control the MMC rectifier station to operate in passive control mode to establish the offshore AC collection bus voltage for large-scale offshore wind farms. 3) Start the grid-connected wind turbines to connect them to the grid and generate electricity, thus realizing the initial power output of the offshore wind farm; 4) Before the injected power exceeds the rated power of the second converter, close the first switch, unlock the diode uncontrolled rectifier unit, then unlock the thyristor rectifier unit, and close the second switch to realize hybrid power transmission. At this time, the onshore converter station is in a hybrid true bipolar topology operation state. 5) After establishing the communication network, offshore wind turbines will be gradually connected to the grid.

[0015] The present invention has the following beneficial effects: In practical operation, the hybrid offshore wind power low-frequency transmission system and its start-up control method described in this invention connect the output end of a large-scale offshore wind farm to an onshore AC collection bus via an offshore AC collection bus and a low-frequency submarine cable. The onshore AC collection bus is connected to the onshore power grid-side AC collection bus via an onshore converter station. The onshore power grid-side AC collection bus is connected to the onshore AC power grid via an onshore AC start-up circuit. The hybrid topology design reduces equipment costs while improving system stability; the true bipolar structure avoids the limitations of pseudo-bipolar systems; and it is suitable for large-scale offshore wind power scenarios, exhibiting good scalability and economy. Attached Figure Description

[0016] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the description of the embodiments of this application 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 based on these drawings without creative effort.

[0017] Figure 1 This is a structural diagram of the present invention; Figure 2 This is a schematic diagram of the startup control process of the present invention; Figure 3 Diagram illustrating the charging process of the maritime communication busbar; Figure 4 This is a schematic diagram of the power flow direction of the system during normal operation.

[0018] Wherein, 1 is the onshore AC power grid; 2 is the onshore AC start-up circuit; 21 is the start-up circuit resistor series current measuring device; 22 is the start-up circuit resistor bypass switch; 23 is the start-up resistor; 24 is the incoming line circuit breaker; 3 is the onshore power grid side AC collecting bus; 41 is the first converter station grid-side transformer; 42 is the second converter station grid-side transformer; 43 is the second switch; 44 is the fourth switch; 5 is the onshore converter station; 51 is the diode uncontrolled rectifier unit; 52 is... Thyristor rectifier unit; 53 is MMC rectifier station; 54 is MMC inverter station; 55 is diode; 61 is first switch; 62 is third switch; 63 is line-side transformer of first converter station; 64 is line-side transformer of second converter station; 7 is onshore AC collection bus; 81 is offshore AC collection bus; 82 is low-frequency submarine cable; 9 is large-scale offshore wind farm; 91 is grid-connected wind turbine; 92 is internal AC collection bus of wind farm; 93 is step-up transformer. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] In the description of this invention, it should be understood that the terms "comprising" and "including" indicate the presence of the described features, integrals, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or collections thereof.

[0021] It should also be understood that the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise.

[0022] It should also be further understood that the term "and / or" as used in this specification and the appended claims refers to any combination and all possible combinations of one or more of the associated listed items, and includes such combinations. For example, A and / or B can represent three cases: A alone, A and B simultaneously, and B alone. Additionally, the character " / " in this invention generally indicates that the preceding and following objects have an "or" relationship.

[0023] It should be understood that although terms such as first, second, third, etc., may be used in the embodiments of the present invention to describe the preset range, these preset ranges should not be limited to these terms. These terms are only used to distinguish the preset ranges from one another. For example, without departing from the scope of the embodiments of the present invention, the first preset range may also be referred to as the second preset range, and similarly, the second preset range may also be referred to as the first preset range.

[0024] Depending on the context, the word "if" as used here can be interpreted as "when," "when," "in response to determination," or "in response to detection." Similarly, depending on the context, the phrase "if determination" or "if detection (of the stated condition or event)" can be interpreted as "when determination," "in response to determination," "when detection (of the stated condition or event)," or "in response to detection (of the stated condition or event)."

[0025] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0026] The accompanying drawings illustrate various structural schematic diagrams according to embodiments disclosed in this invention. These drawings are not to scale, and some details have been enlarged for clarity, and some details may have been omitted. The shapes of the various regions and layers shown in the drawings, as well as their relative sizes and positional relationships, are merely exemplary and may deviate from reality due to manufacturing tolerances or technical limitations. Furthermore, those skilled in the art can design regions / layers with different shapes, sizes, and relative positions as needed.

[0027] refer to Figure 1 The hybrid offshore wind power low-frequency transmission system of the present invention includes a large-scale offshore wind farm 9, an onshore AC collection bus 7, an onshore power grid-side AC collection bus 3, and an onshore AC power grid 1; The output end of the large-scale offshore wind farm 9 is connected to the onshore AC collection bus 7 via the offshore AC collection bus 81 and the low-frequency submarine cable 82. The onshore AC collection bus 7 is connected to the onshore power grid side AC collection bus 3 via the onshore converter station 5. The onshore power grid side AC collection bus 3 is connected to the onshore AC power grid 1 via the onshore AC start-up circuit 2.

[0028] In this embodiment, the large-scale offshore wind farm 9 includes several grid-connected wind turbine units 91, and each grid-connected wind turbine unit 91 is connected to the offshore AC collection bus 81 via the internal AC collection bus 92 of the wind farm and the step-up transformer 93.

[0029] In this embodiment, the onshore converter station 5 includes a diode uncontrolled rectifier unit 51, a thyristor rectifier unit 52, an MMC rectifier station 53, and an MMC inverter station 54. The onshore AC collecting bus 7 is connected to the onshore grid-side AC collecting bus 3 via the diode uncontrolled rectifier unit 51 and the thyristor rectifier unit 52. The onshore AC collecting bus 7 is also connected to the onshore grid-side AC collecting bus 3 via the MMC rectifier station 53, the diode 55, and the MMC inverter station 54.

[0030] In this embodiment, the onshore AC collection bus 7 is connected to the diode uncontrolled rectifier unit 51 via the first switch 61 and the first converter station line-side transformer 63, and the thyristor rectifier unit 52 is connected to the onshore power grid-side AC collection bus 3 via the first converter station grid-side transformer 41 and the second switch 43.

[0031] In this embodiment, the onshore AC collection bus 7 is connected to the MMC rectifier station 53 via the third switch 62 and the line-side transformer 64 of the second converter station, and the MMC inverter station 54 is connected to the onshore grid-side AC collection bus 3 via the grid-side transformer 42 of the second converter station and the fourth switch 44.

[0032] In this embodiment, the onshore AC starting circuit 2 includes a starting circuit resistor series current measuring device 21, a starting circuit resistor bypass switch 22, and a starting resistor 23. The onshore power grid side AC collecting bus 3 is connected to the onshore AC power grid 1 via the starting circuit resistor series current measuring device 21, the starting resistor 23, and the incoming circuit breaker 24. One end of the starting circuit resistor bypass switch 22 is connected to the line between the starting circuit resistor series current measuring device 21 and the onshore power grid side AC collecting bus 3, and the other end of the starting circuit resistor bypass switch 22 is connected to the line between the starting resistor 23 and the incoming circuit breaker 24.

[0033] In this embodiment, both the diode uncontrolled rectifier unit 51 and the thyristor rectifier unit 52 are six-pulse, and can be expanded to twelve-pulse according to the actual needs of the scenario. In this embodiment, the MMC rectifier station 53 and the MMC inverter station 54 form a back-to-back frequency converter. The MMC rectifier station 53 and the MMC inverter station 54 are connected through a common DC bus to convert DC to power frequency AC, supporting bidirectional power flow and reactive power compensation.

[0034] In this embodiment, the low-frequency electrical energy from the large-scale offshore wind farm 9 is collected by the internal AC collection bus 92 of the wind farm, connected to the step-up transformer 93, and then sent to the onshore AC collection bus 7 via the offshore AC collection bus 81 and the low-frequency submarine cable 82. Part of the low-frequency electrical energy at the onshore collection station is rectified by the diode uncontrolled rectifier unit 51 via the first switch 61, then inverted by the thyristor rectifier unit 52 and connected to the second switch 43, and finally connected to the onshore grid-side AC collection bus 3; the other part of the low-frequency electrical energy is connected to the back-to-back frequency converter via the third switch 62, and then connected to the onshore grid-side AC collection bus 3 via the fourth switch 44.

[0035] In this embodiment, the low-frequency submarine cable 82 adopts a single-core or multi-core structure and operates at a frequency of 20Hz or 50 / 3Hz. It is used to transmit the boosted low-frequency electrical energy to land. Compared with the power frequency submarine cable, it can reduce capacitance effect and loss and increase transmission capacity by more than 20%.

[0036] In this embodiment, the diode uncontrolled rectifier unit 51 adopts a six-pulse or twelve-pulse structure, which is composed of multiple diode valve groups connected in series. It is used to rectify low-frequency AC power into DC power, support unidirectional power flow, and reduce harmonic injection.

[0037] In this embodiment, the thyristor rectifier unit 52 also adopts a six-pulse or twelve-pulse structure, which is composed of thyristor valve groups and is used to invert DC power into AC power at the power frequency. Power regulation and fault isolation are achieved through phase control.

[0038] In this embodiment, the system as a whole adopts a hybrid true bipolar topology, wherein the diode uncontrolled rectifier unit 51 and the thyristor rectifier unit 52 constitute the first converter as the positive path; the back-to-back frequency converter constitutes the second converter as the negative path; the two converters share a common ground wire to achieve power balance and ground fault isolation.

[0039] Accordingly, refer to Figure 2 , Figure 3 and Figure 4 The startup control method for the hybrid offshore wind power low-frequency transmission system of the present invention includes the following steps: 1) Switch the onshore converter station 5 to the operating state, close the fourth switch 44, start the MMC inverter station 54, and use the charging resistor in the onshore converter station 5 to charge the DC bus of the second converter using the onshore AC power grid 1. 2) Start the MMC rectifier station 53, close the third switch 62, and after the DC voltage stabilizes, control the MMC rectifier station 53 to operate in passive control mode to establish the offshore AC collection bus voltage of the large-scale offshore wind farm 9. 3) Start the grid-connected wind turbine 91 to connect it to the grid and generate electricity, thereby realizing the initial power output of the offshore wind farm; 4) Before the injected power exceeds the rated power of the second converter, close the first switch 61, unlock the diode uncontrolled rectifier unit 51 in the first converter, then unlock the thyristor rectifier unit 52 in the first converter, and close the second switch 43 to realize hybrid power transmission. At this time, the land-based converter station 5 is in a hybrid true bipolar topology operation state. 5) After establishing the communication network, offshore wind turbines will be gradually connected to the grid.

[0040] The onshore converter station 5 in this invention adopts a hybrid true bipolar topology combining a first converter station and a second converter station. The first converter station consists of a diode uncontrolled rectifier unit 51 and a thyristor rectifier unit 52, while the second converter station consists of back-to-back frequency converters. During startup, the second converter station establishes the voltage of the offshore AC collection bus 81, unlocking and supporting the large-scale wind farm. After establishing the voltage of the offshore AC collection bus 81, the first converter station is unlocked. During normal operation, both the first and second converter stations transmit low-frequency electrical energy from the large-scale wind farm. This invention reduces equipment costs and improves system stability through a hybrid topology design; the true bipolar structure avoids the limitations of pseudo-bipolar systems; it is suitable for large-scale offshore wind power scenarios and has good scalability and economy.

[0041] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and disclosure of the invention. This application is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. The specification and examples are to be considered exemplary only, and the true scope and spirit of the invention are indicated by the following claims.

[0042] It should be understood that the present invention is not limited to the precise structure described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

[0043] The above description is merely a preferred embodiment of the present invention and does not constitute any limitation on the present invention. Any simple modifications, alterations, or equivalent structural changes made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A hybrid offshore wind power low-frequency transmission system, characterized in that, It includes large-scale offshore wind farms (9), onshore AC collection busbars (7), onshore power grid-side AC collection busbars (3) and onshore AC power grids (1); The output end of the large-scale offshore wind farm (9) is connected to the onshore AC collection bus (7) via the offshore AC collection bus (81) and the low-frequency submarine cable (82). The onshore AC collection bus (7) is connected to the onshore power grid side AC collection bus (3) via the onshore converter station (5). The onshore power grid side AC collection bus (3) is connected to the onshore AC power grid (1) via the onshore AC start-up circuit (2).

2. The hybrid offshore wind power low-frequency transmission system according to claim 1, characterized in that, The large-scale offshore wind farm (9) includes several grid-connected wind turbine units (91), each of which is connected to the offshore AC collection bus (81) via the internal AC collection bus (92) and step-up transformer (93) of the wind farm.

3. The hybrid offshore wind power low-frequency transmission system according to claim 1, characterized in that, The onshore converter station (5) includes a diode uncontrolled rectifier unit (51), a thyristor rectifier unit (52), an MMC rectifier station (53), and an MMC inverter station (54). The MMC rectifier station (53) and the MMC inverter station (54) form a back-to-back frequency converter. The onshore AC collection bus (7) is connected to the onshore power grid side AC collection bus (3) via the diode uncontrolled rectifier unit (51) and the thyristor rectifier unit (52). The onshore AC collection bus (7) is connected to the onshore power grid side AC collection bus (3) via the MMC rectifier station (53), the diode (55), and the MMC inverter station (54).

4. The hybrid offshore wind power low-frequency transmission system according to claim 3, characterized in that, The onshore AC collection bus (7) is connected to the diode uncontrolled rectifier unit (51) via the first switch (61) and the line-side transformer (63) of the first converter station.

5. The hybrid offshore wind power low-frequency transmission system according to claim 3, characterized in that, The thyristor rectifier unit (52) is connected to the AC collection bus (3) on the land power grid side via the first converter station grid-side transformer (41) and the second switch (43).

6. The hybrid offshore wind power low-frequency transmission system according to claim 3, characterized in that, The land-based AC collection bus (7) is connected to the MMC rectifier station (53) via the third switch (62) and the line-side transformer (64) of the second converter station.

7. The hybrid offshore wind power low-frequency transmission system according to claim 3, characterized in that, The MMC inverter station (54) is connected to the AC collection busbar (3) on the land power grid side via the second converter station grid-side transformer (42) and the fourth switch (44).

8. The hybrid offshore wind power low-frequency transmission system according to claim 1, characterized in that, The onshore AC starting circuit (2) includes a starting circuit resistor series current measuring device (21), a starting circuit resistor bypass switch (22), and a starting resistor (23). The onshore power grid side AC collecting bus (3) is connected to the onshore AC power grid (1) via the starting circuit resistor series current measuring device (21), the starting resistor (23), and the incoming line circuit breaker (24). One end of the starting circuit resistor bypass switch (22) is connected to the line between the starting circuit resistor series current measuring device (21) and the onshore power grid side AC collecting bus (3), and the other end of the starting circuit resistor bypass switch (22) is connected to the line between the starting resistor (23) and the incoming line circuit breaker (24).

9. The hybrid offshore wind power low-frequency transmission system according to claim 3, characterized in that, Both the diode uncontrolled rectifier unit (51) and the thyristor rectifier unit (52) are six-pulse.

10. A startup control method for a hybrid offshore wind power low-frequency transmission system, characterized in that, include: 1) Switch the onshore converter station (5) to the operating state, close the fourth switch (44), start the MMC inverter station (54), and use the charging resistor in the onshore converter station (5) to charge the DC bus of the back-to-back inverter using the onshore AC power grid (1); 2) Start the MMC rectifier station (53), close the third switch (62), and after the DC voltage stabilizes, control the MMC rectifier station (53) to operate in passive control mode in order to establish the offshore AC collection bus voltage of the large-scale offshore wind farm (9); 3) Start the grid-connected wind turbine (91) to connect the grid-connected wind turbine (91) to the grid for power generation and realize the initial power output of the offshore wind farm; 4) Before the injected power exceeds the rated power of the second converter, close the first switch (61), unlock the diode uncontrolled rectifier unit (51), then unlock the thyristor rectifier unit (52), and close the second switch (43) to realize hybrid power transmission. At this time, the land-based converter station (5) is in a hybrid true bipolar topology operation state. 5) After establishing the communication network, offshore wind turbines will be gradually connected to the grid.

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