Offshore wind power direct current transmission system and fault clearing method thereof

Abstract

The invention discloses an offshore wind power direct current transmission system and a fault clearing method thereof. The system comprises an offshore wind power plant, an offshore sending end converter station, a sending end alternating current power grid, an onshore sending end converter station, an onshore direct current pooling station, an onshore receiving end converter station and a load center alternating current power grid. The offshore sending end converter station adopts a symmetrical monopole wiring form, and the onshore sending end converter station and the onshore receiving end converter station both adopt a symmetrical bipolar wiring form; the direct current side of the offshore sending end converter station is electrically connected with the positive electrode and the negative electrode of the bus bar. The direct current side of the onshore sending end converter station is connected with the positive electrode, the negative electrode and the neutral line of the bus bar. The direct current side of the onshore receiving end converter station is connected with the positive electrode, the negative electrode and the neutral line of the bus bar. Compared with the prior art, the power transmission stability and economical efficiency of the offshore wind power direct-current power transmission system are both considered, and fault ride-through of the offshore wind power flexible direct-current power transmission system is achieved.

Description

Technical Field

[0001] The present invention relates to the field of offshore wind power direct current transmission engineering, 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 huge development potential. Offshore wind power has less negative impact on the environment, has relatively stable wind speed, and has large power generation. Offshore wind power is a trend in the future development of wind power.

[0003] A Chinese patent application with publication number CN117526394A discloses an offshore wind power DC system, which proposes a technical solution in which the electric energy generated by offshore wind power is inverted 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 a DC overhead line.

[0004] However, the patent application has the following technical problems: the offshore converter station and the onshore converter station use the same wiring form: symmetrical monopole or symmetrical bipole. For both the offshore converter station and the onshore converter station, if the DC overhead line fails, the system will lose 100% of the power, seriously affecting the stability of the receiving end power grid. This scheme will not be applicable to the receiving end power grid with a weaker grid. For both the offshore converter station and the onshore converter station, a symmetrical bipole wiring scheme is adopted. There is a DC bias voltage on the valve side of the connecting transformer of the offshore converter station with symmetrical bipole wiring, which makes the platform layout design of the offshore converter station complex, and the neutral line DC submarine cable is expensive, resulting in a high overall cost and poor economy. Secondly, the intermittent and volatile output of offshore wind farms is not taken into account. Since the utilization hours of offshore wind farms are not high, if it is not possible to coordinate the bundling of regulating power sources such as coastal thermal power and nuclear power with offshore wind power, the availability of the offshore wind power flexible DC transmission system will be low and the comprehensive investment cost will be high. 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 object, the present invention adopts the following technical solution.

[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 grid, an onshore sending-end converter station, an onshore direct current collection station, an onshore receiving-end converter station and a load center alternating current grid;

[0008] The onshore DC collection station is equipped with a busbar, and the busbar 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 DC transmission system by adopting the technical solution of single-pole connection in the offshore converter station and symmetrical bipolar 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 pole of the onshore sending-end converter station and the onshore receiving-end converter station is electrically connected to the positive pole of the busbar through a positive DC overhead line, the negative pole of the onshore sending-end converter station and the onshore receiving-end converter station is electrically connected to the negative pole of the busbar through a negative DC overhead line, and the neutral line of the onshore sending-end converter station and the onshore receiving-end converter station is electrically connected to the neutral line of the busbar through a 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 electrodes of the offshore sending-end converter station are electrically connected to the positive and negative electrodes of the busbar respectively through a DC submarine cable; a DC circuit breaker is arranged on the DC submarine cable; and a DC energy consumption device is arranged 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 disposed 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] Since the offshore converter station with symmetrical monopole connection has no neutral line loop, when a monopole grounding fault occurs in the DC overhead line, the neutral line of the onshore converter station with symmetrical bipole connection cannot form a loop, and the fault ride-through of the offshore wind power flexible DC transmission system cannot be achieved. Therefore, in order to achieve the 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-end converter station, all DC circuit breakers are disconnected, DC energy consumption devices are put into operation, the offshore converter station is not locked, and the converter unit with a fault in the onshore sending-end converter station or the onshore receiving-end 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 ground fault returns 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 the set value, the converter unit that still has a 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 converter unit with a fault corresponding to the onshore converter station connected to the fault line is locked, and the converter units with other faults increase the DC voltage to the rated value;

[0026] S4. All DC circuit breakers are closed and DC energy consuming 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., which significantly reduces the investment in offshore wind power transmission; the onshore converter station can achieve only 50% transmission power loss under single-pole fault, with high reliability and friendly to the receiving-end power grid with weak 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 crossing, so that the systems with different connection forms of 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 send them to the receiving power grid at the load center through bundling of direct current overhead lines, which 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 A topological diagram of an offshore wind power direct current transmission system in one embodiment of the present invention;

[0032] Figure 2 The figure is 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 in conjunction with the accompanying drawings and specific implementation methods.

[0034] Embodiment 1

[0035] See also Figure 1 An offshore wind power DC 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 collection station, an onshore receiving-end converter station and a load center AC power grid. The onshore DC collection station is equipped with a busbar, which has a positive pole, a negative pole and a neutral line.

[0036] Among them, due to the limited load-bearing capacity and manufacturing level of the offshore platform, the use of symmetrical bipolar wiring will significantly increase the cost and construction difficulty. Therefore, the offshore converter station adopts a symmetrical monopolar wiring form. Due to the complex topology of the full-bridge MMC, the large size and weight, and the increased cost of the flexible direct current converter valve, the converter unit of the offshore converter station adopts a half-bridge MMC topology.

[0037] The onshore sending-end converter station and the onshore receiving-end converter station adopt symmetrical bipolar wiring, which can significantly improve the reliability of the system. When a single-pole fault occurs, the other pole can still transmit 50% of the power to ensure the normal operation of the system. The onshore sending-end converter station and the onshore receiving-end converter unit adopt a full-bridge half-bridge hybrid MMC topology structure. The full-half-bridge hybrid MMC topology structure not only has the fault clearing capability of the full-bridge submodule, but also has the advantages of low cost and low loss of the half-bridge submodule.

[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 lines 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 lines of the busbar through DC overhead lines, and the AC side is electrically connected to the AC grid at the load center through AC overhead lines.

[0039] Among them, 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 line DC overhead line; both ends of the positive DC overhead line and the negative DC overhead line are equipped with high-speed DC switches HSS, and both ends of the neutral line DC overhead line are equipped with AC circuit breakers BRK. The neutral lines of the onshore sending-end converter station and the onshore receiving-end converter station are equipped with neutral line transfer switches NBS.

[0040] The positive and negative poles of the offshore sending-end converter station are electrically connected to the positive and negative poles of the busbar through a DC submarine cable. A DC circuit breaker DCCB is configured on the DC submarine cable; a DC energy dissipation device is configured between the two DC submarine cables.

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

[0042] Embodiment 2

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

[0044] S1. When a ground fault is detected in the DC overhead line of the onshore receiving-end converter station, all DC circuit breakers are disconnected to cut off the current fed into the fault point by the DC cable; the DC energy consumption device is put into operation, the offshore converter station is not locked, and the electric energy generated by the offshore wind turbine is consumed in the DC energy consumption device after passing through the DC cable. The converter unit with a fault in the onshore sending-end converter station or the onshore receiving-end converter station is transferred to the zero-power control mode. Since the onshore converter station with symmetrical bipolar connection adopts a full half-bridge hybrid MMC structure, the DC overhead line can be de-energized and de-ionized.

[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 exited; 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 through the DC overhead line Flexible DC transmission system is 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 a 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 with other faults increase the DC voltage to the rated value.

[0048] S4. All DC circuit breakers are closed and DC energy consumption devices are shut down. At this time, the electric energy generated by the offshore wind turbine is still inverted into DC through the offshore converter station with symmetrical unipolar connection, collected at the onshore DC collection station, and then transmitted to the receiving end grid of the load center through the DC overhead line. The onshore converter station with symmetrical bipolar connection connected to the fault line switches to unipolar neutral line loop operation, and the symmetrical bipolar converter station at the sending end switches to bipolar power asymmetric operation mode.

[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 in understanding one or more of the various inventive aspects, in the above description of exemplary embodiments of the present invention, various features of the present invention are sometimes grouped together into a single embodiment, figure, or description thereof. However, this disclosed method should not be interpreted as reflecting the intention that the claimed invention requires more features than those expressly recited in each claim. Rather, as reflected in the claims, inventive aspects lie in less than all of the features of the individual embodiments previously disclosed. Therefore, the claims that follow the detailed description are hereby expressly incorporated into the detailed description, with each claim itself serving as a separate embodiment of the present invention.

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

[0052] Those skilled in the art will appreciate that the modules in the devices in 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 in addition they may be divided into a plurality of submodules or subunits or subgroups. Except that at least some of such features and / or processes or units are mutually exclusive, all features disclosed in this specification (including the accompanying claims, abstracts and drawings) and all processes or units of any method or device disclosed in this manner may be combined in any combination. Unless otherwise expressly stated, 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 included in other embodiments but not other features, the combination of features from different embodiments is meant to be within the scope of the present 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. Therefore, a processor with necessary instructions for implementing the method or method elements forms a device for implementing the method or method elements. In addition, 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 combination with hardware or software, or a combination thereof. Thus, the method and apparatus of the present invention, or some aspects or portions of the method and apparatus of the present invention may be in the form of program codes (i.e., instructions) embedded in a tangible medium, such as a floppy disk, a CD-ROM, a hard disk 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 a device for practicing the present invention.

[0056] In the case where 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 transmission mechanism, and includes any information delivery medium. Any combination of the above is 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 is not intended to imply that the objects so described must have a given order in time, space, order, or in any other manner.

[0059] Although the present invention has been described according to a limited number of embodiments, it will be apparent to those skilled in the art, with the benefit of the above description, that other embodiments may be envisioned within the scope of the invention thus described. In addition, it should be noted that the language used in this specification is selected primarily for readability and teaching purposes, rather than for explaining or defining the subject matter of the present invention. Therefore, 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 illustrative, not restrictive, with respect to the scope of the present 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 protection scope of the present invention.

Claims

1. An offshore wind power direct current transmission system, characterized in that: It includes 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, and the busbar 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 line 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 line 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 arranged 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 electrodes of the offshore sending-end converter station are respectively electrically connected to the positive and negative electrodes of the busbar through a DC submarine cable; a DC circuit breaker is arranged on the DC submarine cable; and a DC energy consumption device is arranged 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-end converter station, all DC circuit breakers are disconnected, DC energy consumption devices are put into operation, the offshore converter station is not locked, and the converter unit with a fault in the onshore sending-end converter station or the onshore receiving-end 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 ground fault returns 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 a 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 other faulty converter units increase the DC voltage to the rated value; S4. All DC circuit breakers are closed and DC energy consuming devices are shut down.

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