An offshore wind power direct current transmission system and fault clearing method thereof

Abstract

The present invention discloses an offshore wind power direct current transmission system and a fault clearing method thereof, comprising an offshore wind farm, an offshore sending-end converter station, a sending-end alternating current (AC) grid, an onshore sending-end converter station, an onshore DC collection station, an onshore receiving-end converter station, and a load center AC grid; the offshore sending-end converter station adopts a symmetrical single-pole wiring form, while the onshore sending-end converter station and the onshore receiving-end converter station both adopt a symmetrical bipolar wiring form; the DC side of the offshore sending-end converter station is electrically connected to the positive and negative poles of a busbar; the DC side of the onshore sending-end converter station is electrically connected to the positive, negative, and neutral poles of the busbar; and the DC side of the onshore receiving-end converter station is electrically connected to the positive, negative, and neutral poles of the busbar. Compared with the prior art, the present invention not only takes into account the transmission stability and economy of the offshore wind power direct current transmission system, but also realizes fault ride-through of the offshore wind power flexible direct current transmission system.

Description

Technical Field

[0001] The present invention relates to the field of offshore wind power direct current transmission projects, and in particular to an offshore wind power direct current transmission system and a fault clearing method thereof. Background Art

[0002] With the development and application of wind power technology, and the gradual saturation of onshore wind power development, offshore wind power has enormous development potential. Offshore wind power has less negative impact on the environment, has relatively stable wind speeds, and generates large amounts of power, making it a future trend in wind power development.

[0003] Chinese patent application publication number CN117526394A discloses an offshore wind power DC system. This system proposes a technical solution in which the electricity generated by offshore wind power is converted into DC through an offshore converter station, landed via a DC submarine cable, collected at an onshore DC collection station, and then transmitted to the load center via DC overhead lines.

[0004] However, this patent application suffers from the following technical issues: the offshore converter station and the onshore converter station use the same wiring configuration: symmetrical monopole or symmetrical bipole. If both the offshore and onshore converter stations use the symmetrical monopole wiring scheme, a fault in the DC overhead line will cause a 100% power loss in the system, severely impacting the stability of the receiving grid. This scheme is not suitable for receiving grids with weaker grid structures. If both the offshore and onshore converter stations use the symmetrical bipole wiring scheme, a DC bias voltage will exist on the valve side of the connecting transformer at the offshore converter station. This complicates the layout design of the offshore converter station platform and increases the cost of the neutral DC submarine cable, resulting in a high overall cost and poor economic efficiency. Furthermore, the patent application fails to take into account the intermittent and fluctuating output of offshore wind farms. Due to the low utilization hours of offshore wind farms, if coastal thermal power, nuclear power, and other regulating power sources cannot be coordinated and bundled with offshore wind power, the availability of the offshore wind power flexible DC transmission system will be low, resulting in high overall investment costs. Summary of the Invention

[0005] The present invention provides an offshore wind power direct current transmission system and a fault clearing method thereof to solve the technical problem that the existing offshore wind power direct current transmission system cannot take into account both transmission stability and economy.

[0006] To achieve the above-mentioned purpose, the present invention adopts the following technical solutions.

[0007] On the one hand, an offshore wind power direct current transmission system is provided, comprising an offshore wind farm, an offshore sending-end converter station, a sending-end alternating current (AC) grid, an onshore sending-end converter station, an onshore DC collection station, an onshore receiving-end converter station, and a load center AC grid;

[0008] The onshore DC collection station is equipped with a busbar, which has a positive pole, a negative pole and a neutral line;

[0009] The offshore sending-end converter station adopts a symmetrical monopole connection form, and the onshore sending-end converter station and the onshore receiving-end converter station both adopt a symmetrical bipole connection form;

[0010] The AC side of the offshore sending-end converter station is electrically connected to the offshore wind farm, and the DC side is electrically connected to the positive and negative poles of the busbar respectively;

[0011] The AC side of the onshore sending-end converter station is electrically connected to the sending-end AC power grid, and the DC side is electrically connected to the positive pole, negative pole and neutral line of the busbar respectively;

[0012] The DC side of the onshore receiving-end converter station is electrically connected to the positive pole, negative pole and neutral line of the busbar, respectively, and the AC side is electrically connected to the load center AC power grid.

[0013] This application takes into account both the transmission stability and economy of the offshore wind power direct current transmission system by adopting the technical solution of single-pole connection in the offshore converter station and symmetrical double-pole connection in the onshore converter station.

[0014] In some embodiments, the offshore sending-end converter station adopts a half-bridge MMC topology structure, and the onshore sending-end converter station and the onshore receiving-end converter station both adopt a full-bridge half-bridge hybrid MMC topology structure.

[0015] In some embodiments, the positive poles of the onshore sending-end converter station and the onshore receiving-end converter station are electrically connected to the positive pole of the busbar through the positive DC overhead line, the negative poles of the onshore sending-end converter station and the onshore receiving-end converter station are electrically connected to the negative pole of the busbar through the negative DC overhead line, and the neutral lines of the onshore sending-end converter station and the onshore receiving-end converter station are electrically connected to the neutral line of the busbar through the neutral DC overhead line; both ends of the positive DC overhead line and the negative DC overhead line are equipped with high-speed DC switches, and both ends of the neutral DC overhead line are equipped with AC circuit breakers.

[0016] In some embodiments, neutral line transfer switches are configured on the neutral lines of the onshore sending-end converter station and the onshore receiving-end converter station.

[0017] In some embodiments, the positive and negative poles of the offshore sending-end converter station are electrically connected to the positive and negative poles of the busbar respectively through a DC submarine cable; a DC circuit breaker is configured on the DC submarine cable; and a DC energy consumption device is configured between the two DC submarine cables.

[0018] In some embodiments, the DC energy consumption device is arranged in an offshore converter station or an onshore DC collection station.

[0019] In some embodiments, the onshore sending-end converter station is arranged in an onshore DC collection station.

[0020] In some embodiments, the sending-end AC power grid may be connected to a thermal power plant or a nuclear power plant.

[0021] Because the symmetrical monopole-connected offshore converter station has no neutral line loop, in the event of a monopole grounding fault on the DC overhead line, the neutral line of the symmetrical bipole-connected onshore converter station cannot form a loop, making it impossible to achieve fault ride-through of the offshore wind power flexible DC transmission system. Therefore, to achieve fault ride-through of the offshore wind power flexible DC transmission system, the present invention also provides a fault clearing method for the above-mentioned offshore wind power DC transmission system, comprising the following steps:

[0022] S1. When a ground fault is detected in the DC overhead line of the onshore receiving converter station, all DC circuit breakers are disconnected and DC energy dissipation devices are switched on. The offshore converter station is not locked, and the converter unit with a fault in the onshore sending converter station or the onshore receiving converter station is switched to zero power control mode.

[0023] S2. When it is detected that the fault point of the DC overhead line is freed, the DC voltage of all faulty converter units is increased to the rated value;

[0024] S3. If it is detected that the DC voltage of the DC overhead line with the ground fault has returned to normal, the DC circuit breaker is closed and the DC energy consumption device is shut down;

[0025] If it is detected that the DC overhead line with a ground fault fails to establish a DC voltage, the process returns to step S2. If the number of failures exceeds a set value, the converter unit still experiencing the fault is switched to zero power control mode, and the DC high-speed switches on both sides of the corresponding fault line are disconnected. The converter unit corresponding to the faulty onshore converter station connected to the faulty line is locked, and the DC voltage of the other faulty converter units is increased to the rated value.

[0026] S4. All DC circuit breakers are closed and DC energy consumption devices are shut down.

[0027] The present invention has at least the following technical effects or advantages:

[0028] 1. The present invention can make the offshore converter station have the advantages of compact and lightweight platform, no need for neutral line DC submarine cable, etc., significantly reducing the investment in offshore wind power transmission; the onshore converter station can achieve only 50% transmission power loss in the event of a single-pole fault, with high reliability and friendly to the receiving end power grid with weaker grid structure.

[0029] 2. When the offshore converter station adopts single-pole connection and the onshore converter station adopts symmetrical bipolar connection, when a DC overhead line fault occurs, the offshore wind power flexible DC transmission system can achieve fault ride-through, so that the systems with different connection forms in the offshore converter station and the onshore converter station can operate safely and stably.

[0030] 3. It can realize the integration of regulating power sources such as coastal thermal power and nuclear power with offshore wind power through direct current, and transmit them to the receiving power grid at the load center through bundling of direct current overhead lines. This is conducive to smoothing the unstable output of offshore wind farms, improving the availability of offshore wind power flexible direct current transmission systems, and reducing the overall investment cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a topological diagram of an offshore wind power direct current transmission system according to an embodiment of the present invention;

[0032] Figure 2 Schematic diagram of a flow chart of a fault clearing method in one embodiment of the present invention. DETAILED DESCRIPTION

[0033] In order to better understand the above technical solution, the above technical solution will be described in detail below with reference to the accompanying drawings and specific implementation methods.

[0034] Example 1

[0035] See also Figure 1 An offshore wind power direct current transmission system includes an offshore wind farm, an offshore sending-end converter station, a sending-end AC power grid, an onshore sending-end converter station, an onshore DC busbar, an onshore receiving-end converter station, and a load center AC power grid. The onshore DC busbar is equipped with a busbar having a positive pole, a negative pole, and a neutral line.

[0036] Due to the limited load-bearing capacity and manufacturing capabilities of offshore platforms, adopting a symmetrical bipolar connection significantly increases costs and construction difficulty. Therefore, offshore converter stations adopt a symmetrical monopolar connection. Due to the complexity, size, and weight of the full-bridge MMC topology, which increases the cost of flexible DC converter valves, the converter units of offshore converter stations adopt a half-bridge MMC topology.

[0037] The onshore sending and receiving converter stations utilize symmetrical bipolar wiring, significantly improving system reliability. Even in the event of a single-pole fault, the other pole can still transmit 50% of the power, ensuring normal system operation. The converter units in both the onshore sending and receiving converter stations utilize a full-bridge and half-bridge hybrid MMC topology. This hybrid MMC topology combines the fault-clearing capabilities of full-bridge submodules with the lower cost and reduced losses of half-bridge submodules.

[0038] The use of overhead lines on land can further save transmission corridors and reduce system costs. Therefore, the AC side of the offshore sending-end converter station is electrically connected to the offshore wind farm, and the DC side is electrically connected to the positive and negative poles of the busbar through DC overhead lines. The AC side of the onshore sending-end converter station is electrically connected to the sending-end AC grid through AC overhead lines, and the DC side is electrically connected to the positive, negative, and neutral poles of the busbar through DC overhead lines. The DC side of the onshore receiving-end converter station is electrically connected to the positive, negative, and neutral poles of the busbar through DC overhead lines, and the AC side is electrically connected to the load center AC grid through AC overhead lines.

[0039] The positive poles of the onshore sending and receiving converter stations are electrically connected to the positive pole of the busbar via a positive DC overhead line. The negative poles of the onshore sending and receiving converter stations are electrically connected to the negative pole of the busbar via a negative DC overhead line. The neutral lines of the onshore sending and receiving converter stations are electrically connected to the neutral line of the busbar via a neutral DC overhead line. High-speed DC switches (HSSs) are installed at both ends of the positive and negative DC overhead lines, and AC circuit breakers (BRKs) are installed at both ends of the neutral DC overhead line. A neutral transfer switch (NBS) is installed on the neutral lines of the onshore sending and receiving converter stations.

[0040] The positive and negative poles of the offshore sending converter station are electrically connected to the positive and negative poles of the busbar via a DC submarine cable. A DC circuit breaker (DCCB) is installed on the DC submarine cable, and a DC energy dissipation device is installed between the two DC submarine cables.

[0041] Preferably, the DC energy consumption device is installed in an offshore converter station or an onshore DC collection station. The onshore sending-end converter station is installed in an onshore DC collection station. The sending-end AC power grid can be connected to a thermal power plant or a nuclear power plant.

[0042] Example 2

[0043] See also Figure 2 A method for clearing faults in the above-mentioned offshore wind power direct current transmission system comprises the following steps:

[0044] S1. When a ground fault is detected in the DC overhead line at the onshore receiving converter station, all DC circuit breakers are disconnected, thereby eliminating the current fed into the fault point by the DC cable. The DC energy dissipation device is activated, and the offshore converter station remains open. The power generated by the offshore wind turbine passes through the DC cable and is consumed within the DC energy dissipation device. The faulty converter unit in either the onshore sending or receiving converter station switches to zero-power control mode. Because the onshore converter station with symmetrical bipolar wiring utilizes a full half-bridge hybrid MMC structure, it can de-energize and de-ionize the DC overhead line.

[0045] S2. When it is detected that the fault point of the DC overhead line is freed, the DC voltage of all faulty converter units is increased to the rated value;

[0046] S3. If it is detected that the DC voltage of the DC overhead line with a ground fault returns to normal, it means that the fault is a transient fault, the DC overhead line has normal power transmission conditions, the DC circuit breaker is closed, and the DC energy consumption device is shut down; at this time, the electric energy generated by the offshore wind turbine is re-inverted into DC through the offshore converter station with symmetrical single-pole connection, collected at the onshore DC collection station, and then transmitted to the load center flexible DC transmission system through the DC overhead line and sent to the load center AC receiving power grid.

[0047] If it is detected that the DC overhead line with a ground fault fails to establish a DC voltage, it means that the fault still exists, and the process returns to step S2; if the number of failures exceeds the set value (for example, once), the fault is determined to be a permanent fault. The converter unit that still has the fault is switched to zero power control mode, the DC high-speed switches on both sides of the corresponding fault line are disconnected to isolate the fault line, the converter unit corresponding to the faulty onshore converter station connected to the faulty line is locked, and the converter units of other faulty poles increase the DC voltage to the rated value.

[0048] S4. All DC circuit breakers are closed, and DC energy-consuming devices are deactivated. At this point, the electricity generated by the offshore wind turbines continues to be converted to DC by the symmetrically unipolar offshore converter station, aggregated at the onshore DC aggregation station, and then transmitted to the receiving grid at the load center via DC overhead lines. The onshore converter station with symmetrically bipolar wiring connected to the faulted line switches to unipolar neutral circuit operation, while the symmetrical bipolar converter station at the sending end switches to bipolar asymmetric power operation.

[0049] In the description provided herein, a large number of specific details are described. However, it is understood that embodiments of the present invention can be practiced without these specific details. In some instances, well-known methods, structures, and techniques are not shown in detail so as not to obscure the understanding of this description.

[0050] Similarly, it should be understood that in order to streamline the present disclosure and aid understanding of one or more of the various inventive aspects, in the above description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together into a single embodiment, figure, or description thereof. However, this disclosed method should not be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as reflected in the claims, inventive aspects lie in less than all the features of the individual embodiments disclosed above. Accordingly, the claims that follow the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of the invention.

[0051] Those skilled in the art will appreciate that the modules, units, or groups of devices in the examples disclosed herein may be arranged in the device described in the embodiment, or alternatively may be located in one or more devices different from the devices in the examples. The modules in the aforementioned examples may be combined into one module or further divided into multiple submodules.

[0052] It will be appreciated by those skilled in the art that the modules in the devices of the embodiments may be adaptively changed and arranged in one or more devices different from the embodiments. The modules or units or groups in the embodiments may be combined into one module or unit or group, and furthermore they may be divided into a plurality of submodules or subunits or subgroups. All features disclosed in this specification (including the accompanying claims, abstracts and drawings) and all processes or units of any method or device disclosed herein may be combined in any combination, except that at least some of such features and / or processes or units are mutually exclusive. Unless expressly stated otherwise, each feature disclosed in this specification (including the accompanying claims, abstracts and drawings) may be replaced by an alternative feature providing the same, equivalent or similar purpose.

[0053] Furthermore, those skilled in the art will appreciate that although some embodiments described herein include certain features and not other features included in other embodiments, the combination of features from different embodiments is intended to be within the scope of the invention and to form different embodiments.

[0054] In addition, some of the embodiments are described herein as methods or combinations of method elements that can be implemented by a processor of a computer system or by other devices that perform the functions described. Thus, a processor having the necessary instructions for implementing the method or method element forms a device for implementing the method or method element. Furthermore, the elements described herein of the device embodiments are examples of devices for implementing the functions performed by the elements for the purpose of implementing the invention.

[0055] The various techniques described herein may be implemented in conjunction with hardware or software, or a combination thereof. Thus, the methods and apparatus of the present invention, or certain aspects or portions of the methods and apparatus of the present invention, may take the form of program code (i.e., instructions) embedded in a tangible medium, such as a floppy disk, CD-ROM, hard drive, or any other machine-readable storage medium, wherein when the program is loaded into a machine such as a computer and executed by the machine, the machine becomes an apparatus for practicing the present invention.

[0056] When the program code is executed on a programmable computer, the computing device generally includes a processor, a storage medium readable by the processor (including volatile and non-volatile memory and / or storage elements), at least one input device, and at least one output device. The memory is configured to store the program code; the processor is configured to execute the method of the present invention according to the instructions in the program code stored in the memory.

[0057] By way of example and not limitation, computer-readable media include computer storage media and communication media. Computer-readable media include computer storage media and communication media. Computer storage media stores information such as computer-readable instructions, data structures, program modules, or other data. Communication media generally embodies computer-readable instructions, data structures, program modules, or other data in a modulated data signal such as a carrier wave or other transport mechanism, and includes any information delivery media. Combinations of any of the above are also included within the scope of computer-readable media.

[0058] As used herein, unless otherwise specified, the use of ordinal numbers "first," "second," "third," etc. to describe common objects merely indicates that different instances of similar objects are involved and are not intended to imply that the objects so described must have a given order in time, space, ranking, or in any other manner.

[0059] Although the present invention has been described with respect to a limited number of embodiments, it will be apparent to those skilled in the art, having benefit of the foregoing description, that other embodiments are contemplated within the scope of the invention thus described. Furthermore, it should be noted that the language used in this specification has been selected primarily for readability and didactic purposes, rather than for the purpose of explaining or limiting the subject matter of the present invention. Consequently, many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the appended claims. The disclosure of the present invention is intended to be illustrative rather than restrictive of the scope of the invention, which is defined by the appended claims.

[0060] Finally, it should be noted that the present invention does not explain in detail the common knowledge recognized by technicians in this field. The above is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. An offshore wind power direct current transmission system, characterized in that: Including offshore wind farms, offshore sending-end converter stations, sending-end AC power grids, onshore sending-end converter stations, onshore DC collection stations, onshore receiving-end converter stations and load center AC power grids; The onshore DC collection station is equipped with a busbar, which has a positive pole, a negative pole and a neutral line; The offshore sending-end converter station adopts a symmetrical monopole connection form, and the onshore sending-end converter station and the onshore receiving-end converter station both adopt a symmetrical bipole connection form; The AC side of the offshore sending-end converter station is electrically connected to the offshore wind farm, and the DC side is electrically connected to the positive and negative poles of the busbar respectively; The AC side of the onshore sending-end converter station is electrically connected to the sending-end AC power grid, and the DC side is electrically connected to the positive pole, negative pole and neutral line of the busbar respectively; The DC side of the onshore receiving-end converter station is electrically connected to the positive pole, negative pole and neutral line of the busbar, respectively, and the AC side is electrically connected to the load center AC power grid.

2. The offshore wind power direct current transmission system according to claim 1, characterized in that: The offshore sending-end converter station adopts a half-bridge MMC topology structure, and the onshore sending-end converter station and the onshore receiving-end converter station both adopt a full-bridge half-bridge hybrid MMC topology structure.

3. The offshore wind power direct current transmission system according to claim 1 or 2, characterized in that: The positive poles of the onshore sending-end converter station and the onshore receiving-end converter station are electrically connected to the positive pole of the busbar through the positive DC overhead line, the negative poles of the onshore sending-end converter station and the onshore receiving-end converter station are electrically connected to the negative pole of the busbar through the negative DC overhead line, and the neutral lines of the onshore sending-end converter station and the onshore receiving-end converter station are electrically connected to the neutral line of the busbar through the neutral DC overhead line; both ends of the positive DC overhead line and the negative DC overhead line are equipped with high-speed DC switches, and both ends of the neutral DC overhead line are equipped with AC circuit breakers.

4. The offshore wind power direct current transmission system according to claim 3, characterized in that: Neutral line transfer switches are provided on the neutral lines of the onshore sending-end converter station and the onshore receiving-end converter station.

5. The offshore wind power direct current transmission system according to claim 1 or 2, characterized in that: The positive and negative poles of the offshore sending-end converter station are electrically connected to the positive and negative poles of the busbar respectively through a DC submarine cable; a DC circuit breaker is provided on the DC submarine cable; and a DC energy consumption device is provided between the two DC submarine cables.

6. The offshore wind power direct current transmission system according to claim 5, characterized in that: The DC energy consumption device is arranged in an offshore converter station or an onshore DC collection station.

7. The offshore wind power direct current transmission system according to claim 1 or 2, characterized in that: The onshore sending-end converter station is arranged in an onshore DC collection station.

8. The offshore wind power direct current transmission system according to claim 1 or 2, characterized in that: The sending-end AC power grid includes a thermal power plant or a nuclear power plant.

9. A fault clearing method for an offshore wind power direct current transmission system according to any one of claims 1 to 5, characterized in that: The steps include: S1. When a ground fault is detected in the DC overhead line of the onshore receiving converter station, all DC circuit breakers are disconnected and DC energy dissipation devices are switched on. The offshore converter station is not locked, and the converter unit with a fault in the onshore sending converter station or the onshore receiving converter station is switched to zero power control mode. S2. When it is detected that the fault point of the DC overhead line is freed, the DC voltage of all faulty converter units is increased to the rated value; S3. If it is detected that the DC voltage of the DC overhead line with the ground fault has returned to normal, the DC circuit breaker is closed and the DC energy consumption device is shut down; If it is detected that the DC overhead line with a ground fault fails to establish a DC voltage, the process returns to step S2; if the number of failures exceeds a set value, the converter unit still having the fault is switched to a zero power control mode, and the DC high-speed switches on both sides of the corresponding fault line are disconnected; The faulty converter unit corresponding to the onshore converter station connected to the faulty line is locked, and the DC voltage of other faulty converter units is increased to the rated value; S4. All DC circuit breakers are closed and DC energy consumption devices are shut down.

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