A land-based wind power transmission system

Abstract

This invention discloses an onshore wind power transmission system to solve the problems of harmonic resonance and reactive power transmission in existing related technologies. The onshore wind power transmission system includes a wind power sending-end unit and a wind power receiving-end unit. The sending-end unit includes a DC transformer and two or more wind turbines. Each wind turbine has an internal turbine-side converter and a corresponding low-voltage circuit breaker. Each pole of the DC transformer has a DC circuit breaker installed. For each wind turbine, the generated electricity is rectified into a first DC current by the turbine-side converter and then connected in parallel to the DC transformer via the low-voltage circuit breaker to be stepped up to a second DC current. This second DC current is then sent to the wind power receiving-end unit via the bipolar DC circuit breaker of the DC transformer. The receiving-end unit includes a receiving-end converter station, a connecting transformer, and a starting circuit. After the second DC current is rectified at the receiving-end converter station, it is stepped up again by the connecting transformer and then connected to the AC power grid via the starting circuit.

Description

Technical Field

[0001] This invention relates to the field of wind power transmission technology, and more particularly to an onshore wind power transmission system. Background Technology

[0002] Wind power bases, by concentrating advantageous resources, can fully leverage economies of scale and reduce power generation costs, making them an important direction for the large-scale development of wind power. Currently, wind power systems mainly employ two methods: AC collection-AC transmission and AC collection-DC transmission. However, large-scale wind power bases involve long collection distances, and using the AC collection-AC transmission method would lead to significant problems such as harmonic resonance and reactive power transmission. On the other hand, using the AC collection-DC transmission method presents the issues of multiple power conversion stages and high system costs. Summary of the Invention

[0003] This invention provides an onshore wind power transmission system to solve or partially solve the problems of AC collection and transmission such as harmonic resonance and reactive power transmission in existing related technologies.

[0004] This invention provides an onshore wind power transmission system, which includes a wind power sending-end unit and a wind power receiving-end unit; wherein,

[0005] The wind power transmission unit includes a DC transformer and two or more wind turbine generators; each wind turbine generator is equipped with an organic side converter, and each wind turbine generator corresponds to a low-voltage circuit breaker; a DC circuit breaker is installed on each of the two poles of the output end of the DC transformer.

[0006] For each of the wind turbine generators, the electrical energy generated by the wind turbine generator is rectified into first DC power by the turbine-side converter and then connected in parallel to the DC transformer via the low-voltage circuit breaker.

[0007] The first DC power is stepped up to the second DC power through the DC transformer, and the second DC power is sent to the wind power receiving unit through the double-pole DC circuit breaker of the DC transformer.

[0008] The wind power receiving-end unit includes a receiving-end converter station, a connecting transformer, and a starting circuit;

[0009] After the second DC power is converted by the receiving-end converter station, it is first boosted by the connecting transformer, then passed through the starting circuit, and finally connected to the AC power grid.

[0010] Optionally, the mechanical parts of each wind turbine are 690V AC, and the electrical energy generated by the wind turbine is rectified into ±550V DC by the turbine-side converter.

[0011] Optionally, the ±550V first DC power obtained after rectification by the machine-side converter is connected in parallel to the input terminal of the DC transformer via the low-voltage circuit breaker; the input terminal of the DC transformer corresponds to the low-voltage side, and the output terminal corresponds to the high-voltage side.

[0012] The low-voltage side of the DC transformer boosts the first DC voltage of ±550V to the second DC voltage of ±30kV, and outputs it through the high-voltage side of the DC transformer.

[0013] Optionally, the DC transformer is composed of multiple DC converter units, which are connected and combined in series and parallel.

[0014] The ±550V first DC power is boosted and rectified by each of the DC-DC converter units to output rectified DC power.

[0015] The voltages of each of the aforementioned DC rectified voltages are summed to obtain a second DC voltage of ±30kV.

[0016] Optionally, each of the DC transformer units consists of an IGBT module, a high-frequency transformer, and a rectifier and filter output;

[0017] In each of the DC transformer units, the ±550V first DC power is first inverted by the IGBT module, then stepped up by the high-frequency transformer, and then rectified and filtered by the rectifier and filter output to output rectified DC power.

[0018] Optionally, the receiving-end converter station adopts a hybrid MMC converter with DC fault clearing capability;

[0019] The ±30kV second DC power is commutated by the full-bridge and half-bridge hybrid MMC converter, then undergoes secondary voltage boosting through the connecting transformer to obtain 35kV boosted AC power, which then passes through the starting circuit and is finally connected to the AC power grid via 35kV power frequency AC transmission.

[0020] Optionally, the bipolar DC circuit breaker at the sending end is connected to both ends of the full-bridge and half-bridge hybrid MMC converter at the receiving end via ±30kV DC overhead lines.

[0021] When a fault occurs in the ±30kV DC overhead line, the full-bridge and half-bridge hybrid MMC converter works together with the bipolar DC circuit breaker to clear the DC line fault.

[0022] Optionally, the bipolar DC circuit breaker is a positive DC circuit breaker and a negative DC circuit breaker; both the bipolar DC circuit breaker and the full-bridge / half-bridge hybrid MMC converter are symmetrical unipolar topologies.

[0023] When a fault occurs in the ±30kV DC overhead line, the line protection of the wind power sending-end unit and the line protection of the wind power receiving-end unit detect the fault and each executes a fault clearing action to achieve coordinated fault clearing at both the sending and receiving ends of the DC line; wherein,

[0024] The line protection of the wind power sending unit generates a trip command and simultaneously sends the trip command to the positive DC circuit breaker and the negative DC circuit breaker to control the tripping of the positive DC circuit breaker and the negative DC circuit breaker.

[0025] The line protection of the wind power receiving unit generates a voltage control command and sends the voltage control command to the full-bridge and half-bridge hybrid MMC converter so that the full-bridge and half-bridge hybrid MMC converter controls the positive and negative voltages of the receiving end DC side to below 0.

[0026] Optionally, after the sending and receiving ends have completed the fault clearing action, and after the deionization occurs within the preset recovery time, the full-bridge and half-bridge hybrid MMC converter restores the positive and negative voltages on the DC side of the receiving end to ±30kV, and the positive DC circuit breaker and the negative DC circuit breaker are simultaneously re-closed, and the onshore wind power transmission system resumes operation.

[0027] Optionally, when the system capacity is greater than or equal to a preset capacity threshold, both the bipolar DC circuit breaker and the full-bridge / half-bridge hybrid MMC converter adopt a true bipolar topology.

[0028] When one of the poles in the bipolar circuit fails, the faulty pole enters a fault-crossing state. The DC circuit breaker on the same pole as the faulty pole and the full-bridge / half-bridge hybrid MMC converter work together to clear the DC line fault. One of the poles in the bipolar circuit is either the positive pole or the negative pole.

[0029] The operation of the line on the pole that did not experience a fault is unaffected.

[0030] As can be seen from the above technical solutions, the present invention has the following advantages:

[0031] An onshore wind power transmission system is provided. The onshore wind power transmission system includes a wind power sending-end unit and a wind power receiving-end unit. The sending-end unit includes a DC transformer and two or more wind turbine generators. Each wind turbine generator is equipped with a generator-side converter and a corresponding low-voltage circuit breaker. Each pole of the DC transformer is equipped with a DC circuit breaker. For each wind turbine generator, the generated electricity is rectified into a first DC current by the generator-side converter and then connected in parallel to the DC transformer via the low-voltage circuit breaker to be stepped up to a second DC current. This second DC current is then sent to the wind power receiving-end unit via the bipolar DC circuit breaker of the DC transformer. The receiving-end unit includes a receiving-end converter station, a connecting transformer, and a starting circuit. After the second DC current is rectified at the receiving-end converter station, it is stepped up again by the connecting transformer and then connected to the AC power grid via the starting circuit. The wind turbine outputs DC, which is then boosted to a medium-to-high voltage level before being transmitted over long-distance overhead lines. This not only reduces intermediate AC-DC conversion stages and effectively solves traditional AC collection and transmission problems such as harmonic resonance and reactive power transmission, but also lowers power generation costs. Furthermore, the all-DC transmission method offers higher transmission efficiency and reliability. Attached Figure Description

[0032] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0033] Figure 1 A schematic diagram of the circuit principle structure of an onshore wind power transmission system;

[0034] Figure 2 This is a schematic diagram of the circuit principle structure for side rectification of a wind turbine generator set;

[0035] Figure 3 This is a schematic diagram of the circuit principle structure of a DC transformer unit for step-up rectification. Detailed Implementation

[0036] This invention provides an onshore wind power transmission system to solve or partially solve the problems of AC collection and transmission such as harmonic resonance and reactive power transmission in existing related technologies.

[0037] To make the objectives, features, and advantages of this invention more apparent and understandable, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0038] As an example, taking onshore wind power transmission as a practical application scenario, wind power generation systems currently mainly employ two methods: AC collection-AC transmission and AC collection-DC transmission. However, large-scale wind power bases have long collection distances, and using the AC collection-AC transmission method would lead to prominent problems such as harmonic resonance and reactive power transmission. On the other hand, using the AC collection-DC transmission method results in more power conversion stages and higher system costs.

[0039] To effectively address the current issues of harmonic resonance and reactive power transmission in onshore wind power transmission, and to achieve efficient grid connection, a novel onshore wind power transmission topology needs to be proposed to reduce the overall cost of the onshore wind power transmission system while ensuring stable transmission of onshore wind power.

[0040] Therefore, one of the core inventive points of this invention is: based on a completely new technical approach, it organically combines DC power transmission technology with DC-type wind turbine units to propose an onshore wind power all-DC power transmission system. The sending end of the onshore wind power transmission system includes multiple wind turbine units and turbine-side converters. The low-voltage DC output from the DC-type wind turbine units is boosted and collected by a DC converter, and then connected to the receiving end's flexible DC converter station via a long-distance overhead line equipped with a DC circuit breaker. The receiving end's flexible DC converter station uses a hybrid MMC (Modular Multilevel Converter) converter with DC fault clearing capabilities. The wind turbine output is DC, which is boosted to a medium-to-high-level DC voltage and finally transmitted via a long-distance overhead line. This not only reduces intermediate AC-DC conversion stages and effectively solves traditional AC collection and transmission problems such as harmonic resonance and reactive power transmission, but also reduces power generation costs. Furthermore, the all-DC transmission method has higher transmission efficiency and reliability.

[0041] Reference Figure 1 The diagram shows a schematic diagram of the circuit principle structure of an onshore wind power (all DC) transmission system provided by an embodiment of the present invention.

[0042] Overall, onshore wind power transmission systems can be mainly divided into wind power sending-end units (corresponding to the sending end) and wind power receiving-end units (corresponding to the receiving end). The wind power sending-end unit mainly includes a DC / DC (Direct Current to Direct Current Converter) transformer and two or more wind turbine generators (for simplicity, ...). Figure 1 (Two permanent magnet direct-drive generators are used as an example for illustration). Each wind turbine is equipped with an organic-side converter (for ease of explanation, in...). Figure 1 (This is shown separately from the wind turbine unit). Each wind turbine unit corresponds to one low-voltage circuit breaker.

[0043] For each wind turbine, the electrical energy generated by the turbine is rectified into first DC power by the turbine-side converter and then connected in parallel to a DC transformer via a low-voltage circuit breaker. The first DC power is then stepped up to second DC power by the DC transformer, and the second DC power is sent to the wind power receiving-end unit via the double-pole DC circuit breaker of the DC transformer.

[0044] Combination Figure 1 , Figure 2 A schematic diagram of the circuit principle structure for side rectification in a wind turbine generator is shown. Figure 1 Taking the permanent magnet direct drive generator 2 in the example, the mechanical part of each wind turbine is 690V AC, and the electrical energy generated by the wind turbine is rectified into ±550V DC by the machine-side converter.

[0045] Next, the ±550V first DC power, rectified by two generator-side converters (or multiple generator-side converters in different application scenarios), is connected in parallel to the input terminal of the DC transformer (corresponding to the low-voltage side) via a low-voltage circuit breaker. The low-voltage side of the DC transformer steps up the ±550V first DC power to a ±30kV second DC power, which is then output via the high-voltage side of the DC transformer (or, in other words, stepped up to ±30kV on the high-voltage side). A DC circuit breaker is installed on each pole (positive and negative) of the DC transformer's output terminal (corresponding to the high-voltage side). This power is then connected to the receiving-end converter station of the wind power receiving unit via a long-distance overhead line.

[0046] from Figure 1 As can be seen, the DC transformer is composed of multiple DC converter units (such as module1~module12), and these multiple DC converter units are connected and combined in series and parallel.

[0047] During the boost process, the ±550V first DC power is first boosted and rectified by each DC converter unit to output rectified DC power. Then, the voltages of each rectified DC power are summed to obtain the ±30kV second DC power.

[0048] More specifically, Figure 3 A schematic diagram of the circuit principle structure of a DC transformer unit is shown.

[0049] Taking a DC transformer with 12 DC transformer units as an example, combined with... Figure 3 It can be seen that each DC transformer unit mainly consists of an IGBT (Insulated Gate Bipolar Transistor) module, a high-frequency transformer, and a rectifier and filter output.

[0050] In each DC transformer unit, the ±550V DC power is first inverted by the IGBT module, then stepped up by the high-frequency transformer, and then rectified and filtered to output rectified DC power.

[0051] More specifically, the DC transformer unit has a ±550V DC input, and the input terminal consists of IGBT modules. First, the ±550V DC is inverted into high-frequency low-voltage AC by the IGBT modules, and then stepped up to high-frequency high-voltage AC by the high-frequency transformer. After rectification and filtering (diode rectifier), the output is ±2.5kV DC.

[0052] This embodiment of the invention provides 12 DC transformer units, each of which can provide a stepped-up DC voltage of ±2.5kV. By connecting these 12 DC / DC converter units in series and parallel, a second DC voltage of ±30kV can be output.

[0053] It is understood that the number of DC transformer units does not necessarily have to be 12, as long as the final output DC voltage is ±30kV. Twelve units are used here because the high-voltage side output of a single DC transformer unit is ±2.5kV, which, when accumulated, equals ±30kV. For example, assuming ±550V DC power can be processed by a single DC converter unit to output 3kV DC power, then only 10 DC converter units are needed to obtain a total DC power of ±30kV. It is understood that this invention does not impose any limitations on this.

[0054] The wind power receiving-end unit includes the receiving-end converter station, the connecting transformer, and the starting circuit.

[0055] The receiving-end converter station adopts a hybrid MMC topology with DC fault clearing capability, namely a hybrid MMC converter with full-bridge and half-bridge configuration.

[0056] An MMC consists of multiple sub-modules (SMs). Each sub-module typically contains a half-bridge or full-bridge inverter and an energy storage capacitor. These sub-modules are connected in series to form a phase arm. A typical MMC includes three phase arms, corresponding to phases A, B, and C in a three-phase AC system.

[0057] The hybrid MMC converter (half-bridge and half-bridge) provided in this embodiment of the invention is mainly composed of two types of submodules: HBSM (Half-Bridge Submodule) and FBSM (Full-Bridge Submodule). The HBSM consists of two IGBTs and one energy storage capacitor. The FBSM consists of four IGBTs and one energy storage capacitor.

[0058] In other words, the basic topology of a hybrid MMC converter at the receiving end mainly includes two types of submodules: full-bridge submodules and half-bridge submodules. Thus, in the event of a fault, the full-bridge submodule can output a negative level, working in conjunction with the half-bridge submodule to control the DC-side voltage below 0. By using half-bridge submodules in the topology of the hybrid MMC converter, compared to using only full-bridge submodules, the number of power devices can be reduced while achieving DC-side fault ride-through, thus lowering the equipment's investment cost.

[0059] The full-bridge and half-bridge hybrid MMC converter connects bipolarly on the DC side to the DC circuit breaker at the sending end, receiving ±30kV secondary DC power. On the AC side, it connects to a connecting transformer and a starting circuit, then is transmitted to the main power grid via 35kV AC power. In other words, after the secondary DC power is converted at the receiving-end converter station, it undergoes a secondary voltage boost through the connecting transformer, then passes through the starting circuit, and finally connects to the AC power grid.

[0060] A connecting transformer (also known as a coupling transformer or a connecting transformer) can further boost the voltage transmitted at the sending end to achieve voltage transformation, thereby reducing line losses during long-distance power transmission.

[0061] A start-up circuit enables the system to start smoothly and reliably after power-on or reset.

[0062] More specifically, the ±30kV second DC power is converted by the full-bridge and half-bridge hybrid MMC converter, then first undergoes secondary voltage boosting through the connecting transformer to obtain 35kV boosted AC power, then passes through the starting circuit, and finally is connected to the AC power grid via 35kV power frequency AC transmission.

[0063] The sending-end bipolar DC circuit breaker is connected to both ends of the receiving-end full-bridge and half-bridge hybrid MMC converter via ±30kV DC overhead lines. When a fault occurs in the ±30kV DC overhead line, the full-bridge and half-bridge hybrid MMC converter works together with the bipolar DC circuit breaker to clear the DC line fault.

[0064] As can be seen from the foregoing, the bipolar DC circuit breaker referred to in this invention includes both a positive DC circuit breaker and a negative DC circuit breaker. In one optional embodiment, both the bipolar DC circuit breaker and the full-bridge / half-bridge hybrid MMC converter are as follows... Figure 1 The symmetrical unipolar topology shown.

[0065] Symmetric unipolar topology is a type of topology used in power electronic converters, particularly in multilevel converters (such as the hybrid full-bridge and half-bridge MMC converter described in this invention). This topology is characterized by an equal number of submodules in each phase arm, and each submodule having the same voltage level.

[0066] When a fault occurs in the ±30kV DC overhead line, the line protection of the wind power sending-end unit and the line protection of the wind power receiving-end unit detect the fault and each executes a fault clearing action to achieve joint coordination in clearing the DC line faults at the sending and receiving ends.

[0067] Specifically, the line protection of the wind power sending unit generates a trip command and simultaneously sends the trip command to both the positive and negative DC circuit breakers to control the tripping of both the positive and negative DC circuit breakers.

[0068] The line protection of the wind power receiving unit generates a voltage control command and sends the voltage control command to the full-bridge and half-bridge hybrid MMC converter so that the full-bridge and half-bridge hybrid MMC converter controls the positive and negative voltages of the receiving end DC side to below 0.

[0069] Furthermore, after the sending and receiving ends have completed the fault clearing action, and after the deionization has been completed within a preset recovery time (e.g., several hundred milliseconds), the full-bridge and half-bridge hybrid MMC converter will restore the positive and negative voltages on the DC side of the receiving end to ±30kV. The positive DC circuit breaker and the negative DC circuit breaker will be closed simultaneously, and the onshore wind power transmission system will resume operation.

[0070] In this embodiment of the invention, a symmetrical unipolar topology is used at the transmitting and receiving ends because the system capacity is relatively small, and the cost of using a symmetrical unipolar topology is low. However, when the system capacity is large, continuing to use a symmetrical unipolar topology may have a significant impact on the system should a DC line fault occur. In this case, a true bipolar scheme can be considered. In this situation, if one pole fails, only that pole will enter a fault-crossing state. Only the DC circuit breaker of that pole needs to be tripped, and only the MMC control voltage of that pole needs to be lowered below 0. The operation of the other pole will not be affected.

[0071] In specific implementations, when the system capacity is greater than or equal to the preset capacity threshold, both the bipolar DC circuit breaker and the full-bridge / half-bridge hybrid MMC converter adopt a true bipolar topology.

[0072] When one pole of the bipolar circuit fails, the faulty pole enters a fault-crossing state. The DC circuit breaker on the same pole as the faulty pole and the full-bridge / half-bridge hybrid MMC converter work together to clear the DC line fault. One of the bipolar poles is either the positive pole or the negative pole.

[0073] The operation of the line on the pole that did not experience a fault is unaffected.

[0074] For example, when the system capacity is large, both the bipolar DC circuit breaker and the full-bridge / half-bridge hybrid MMC converter adopt a true bipolar topology. When a fault occurs on the positive pole of the bipolar circuit, the positive pole enters a fault-crossing state. The DC circuit breaker on the same pole as the fault (i.e., both are positive) and the full-bridge / half-bridge hybrid MMC converter (the end of the full-bridge / half-bridge hybrid MMC converter connected to the positive pole) work together to clear the DC line fault. The operation of the negative pole, which is not faulty, is unaffected.

[0075] It should be noted that, in order to enable those skilled in the art to better distinguish data of the same type but with different actual meanings, the embodiments of the present invention use "first" and "second" to distinguish and describe some technical features. "First" and "second" are only used to distinguish data and have no other special meaning. It is understood that the present invention does not impose any limitations on them.

[0076] In this embodiment of the invention, based on a novel technical approach, a fully DC transmission system for onshore wind power is proposed, organically combining DC power transmission technology with DC-type wind turbines. The sending end of the onshore wind power transmission system includes multiple wind turbines and turbine-side converters. The low-voltage DC output from the DC-type wind turbines is boosted and collected by a DC converter, and then connected to the receiving end's flexible DC converter station via a long-distance overhead line equipped with a DC circuit breaker. The receiving end's flexible DC converter station employs a hybrid MMC converter with DC fault clearing capabilities (full-bridge and half-bridge). The wind turbine output is DC, which is boosted to a medium-to-high-level DC voltage before being transmitted via a long-distance overhead line. This not only reduces intermediate AC-DC conversion stages, effectively solving traditional AC collection and transmission problems such as harmonic resonance and reactive power transmission, but also reduces power generation costs. Furthermore, the fully DC transmission method offers higher transmission efficiency and reliability.

[0077] In the embodiments provided by this invention, it should be understood that the disclosed system can be implemented in other ways. For example, the device embodiments described above are merely illustrative. For instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be through some interfaces, indirect coupling or communication connection between devices or units, and may be electrical, mechanical, or other forms.

[0078] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0079] Furthermore, the functional units in the various embodiments of the present invention can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit.

[0080] If the integrated unit is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or all or part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of the present invention. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0081] The above-described embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. An onshore wind power transmission system, characterized in that, The onshore wind power transmission system includes a wind power sending-end unit and a wind power receiving-end unit; wherein... The wind power transmission unit includes a DC transformer and two or more wind turbine generators; each wind turbine generator is equipped with an organic side converter, and each wind turbine generator corresponds to a low-voltage circuit breaker; a DC circuit breaker is installed on each of the two poles of the output end of the DC transformer. For each of the wind turbine generators, the electrical energy generated by the wind turbine generator is rectified into first DC power by the turbine-side converter and then connected in parallel to the DC transformer via the low-voltage circuit breaker. The first DC power is stepped up to the second DC power through the DC transformer, and the second DC power is sent to the wind power receiving unit through the double-pole DC circuit breaker of the DC transformer. The wind power receiving-end unit includes a receiving-end converter station, a connecting transformer, and a starting circuit; After the second DC power is converted by the receiving-end converter station, it is first boosted by the connecting transformer, then passed through the starting circuit, and finally connected to the AC power grid. The receiving-end converter station adopts a full-bridge and half-bridge hybrid MMC converter with DC fault clearing capability; the bipolar DC circuit breakers are a positive DC circuit breaker and a negative DC circuit breaker; the sending-end bipolar DC circuit breaker is connected to both ends of the receiving-end full-bridge and half-bridge hybrid MMC converter through ±30kV DC overhead lines. When a fault occurs in the ±30kV DC overhead line, the full-bridge / half-bridge hybrid MMC converter and the bipolar DC circuit breaker work together to clear the DC line fault. This includes: the line protection of the wind power sending unit and the line protection of the wind power receiving unit detect the fault and each executes a fault clearing action to achieve coordinated fault clearing at both the sending and receiving ends of the DC line; wherein, the line protection of the wind power sending unit generates a trip command and simultaneously sends the trip command to both the positive and negative DC circuit breakers to control the tripping of both the positive and negative DC circuit breakers; the line protection of the wind power receiving unit generates a voltage control command and sends the voltage control command to the full-bridge / half-bridge hybrid MMC converter to control the positive and negative voltages on the receiving end DC side to below 0.

2. The onshore wind power transmission system according to claim 1, characterized in that, The mechanical parts of each wind turbine are 690V AC, and the electrical energy generated by the wind turbine is rectified into ±550V DC by the turbine-side converter.

3. The onshore wind power transmission system according to claim 2, characterized in that, The ±550V first DC power obtained after rectification by the machine-side converter is connected in parallel to the input terminal of the DC transformer via the low-voltage circuit breaker; the input terminal of the DC transformer corresponds to the low-voltage side, and the output terminal corresponds to the high-voltage side. The low-voltage side of the DC transformer boosts the first DC voltage of ±550V to the second DC voltage of ±30kV, and outputs it through the high-voltage side of the DC transformer.

4. The onshore wind power transmission system according to claim 3, characterized in that, The DC transformer is composed of multiple DC converter units, which are connected and combined in series and parallel. The ±550V first DC power is boosted and rectified by each of the DC-DC converter units to output rectified DC power. The voltages of each of the aforementioned DC rectified voltages are summed to obtain a second DC voltage of ±30kV.

5. The onshore wind power transmission system according to claim 4, characterized in that, Each of the aforementioned DC transformer units consists of an IGBT module, a high-frequency transformer, and a rectified and filtered output; In each of the DC transformer units, the ±550V first DC power is first inverted by the IGBT module, then stepped up by the high-frequency transformer, and then rectified and filtered by the rectifier and filter output to output rectified DC power.

6. The onshore wind power transmission system according to any one of claims 3 to 5, characterized in that, The ±30kV second DC power is commutated by the full-bridge and half-bridge hybrid MMC converter, then undergoes secondary voltage boosting through the connecting transformer to obtain 35kV boosted AC power, which then passes through the starting circuit and is finally connected to the AC power grid via 35kV power frequency AC transmission.

7. The onshore wind power transmission system according to claim 1, characterized in that, Both the bipolar DC circuit breaker and the full-bridge / half-bridge hybrid MMC converter have symmetrical unipolar topologies.

8. The onshore wind power transmission system according to claim 1, characterized in that, After the sending and receiving ends complete the fault clearing action, and after the deionization occurs within the preset recovery time, the full-bridge and half-bridge hybrid MMC converter restores the positive and negative voltages on the DC side of the receiving end to ±30kV. The positive DC circuit breaker and the negative DC circuit breaker are simultaneously re-closed, and the onshore wind power transmission system resumes operation.

9. The onshore wind power transmission system according to claim 1, characterized in that, When the system capacity is greater than or equal to the preset capacity threshold, both the bipolar DC circuit breaker and the full-bridge / half-bridge hybrid MMC converter adopt a true bipolar topology. When one of the poles in the bipolar circuit fails, the faulty pole enters a fault-crossing state. The DC circuit breaker on the same pole as the faulty pole and the full-bridge / half-bridge hybrid MMC converter work together to clear the DC line fault. One of the poles in the bipolar circuit is either the positive pole or the negative pole. The operation of the line on the pole that did not experience a fault is unaffected.

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