Land wind power transmission system

Abstract

The invention discloses a land wind power transmission system, which is used for solving the problems of alternating current collection and transmission such as harmonic resonance and reactive power transmission in the prior art. The land wind power transmission system comprises a wind power sending end unit and a wind power receiving end unit. The wind power sending end unit comprises a direct current transformer and more than two wind turbine generators; a generator-side converter is arranged in each wind generator set and corresponds to one low-voltage circuit breaker; two poles of the direct-current transformer are respectively provided with a direct-current circuit breaker; for each wind power generation set, electric energy generated by the wind power generation set is rectified into first direct current through a machine-side converter, and the first direct current is connected into a direct-current transformer in parallel through a low-voltage circuit breaker to be boosted into second direct current which is sent into a wind power receiving end unit through a bipolar direct-current circuit breaker of the direct-current transformer; the wind power receiving end unit comprises a receiving end converter station, a connection transformer and a starting loop; and the second direct current is subjected to secondary boosting through the connection transformer after being subjected to current conversion through the receiving-end converter station, and is accessed to the alternating-current power grid through the starting loop.

Description

Technical Field

[0001] The present invention relates to the technical field of wind power transmission, and in particular to an onshore wind power transmission system. Background Art

[0002] Wind power generation bases gather advantageous resources, which can give full play to the scale effect and reduce the cost of power generation. It is an important direction for the large-scale development of wind power generation. At present, there are two main types of wind power generation systems: AC collection-AC transmission and AC collection-DC transmission. However, large wind power generation bases have long collection distances. If the AC collection-AC transmission method is adopted, problems such as harmonic resonance and reactive power transmission will be prominent. If the AC collection-DC transmission method is adopted, there will be many power conversion links and high system costs. Summary of the invention

[0003] The present invention provides an onshore wind power transmission system for solving or partially solving the problems of AC collection and transmission such as harmonic resonance and reactive power transmission existing in the prior art.

[0004] The present invention provides an onshore wind power transmission system, which comprises a wind power sending end unit and a wind power receiving end unit; wherein:

[0005] The wind power sending end unit includes a DC transformer and two or more wind turbines; each wind turbine is provided with an organic side converter, and each wind turbine corresponds to a low voltage circuit breaker; a DC circuit breaker is installed at each bipolar output end of the DC transformer;

[0006] For each wind turbine generator set, the electric energy generated by the wind turbine generator set is rectified into a first direct current through the machine-side converter, and connected in parallel to the direct current transformer through the low-voltage circuit breaker;

[0007] The first direct current is boosted into a second direct current by the direct current transformer, and the second direct current is sent to the wind power receiving unit through the bipolar direct current circuit breaker of the direct current transformer;

[0008] The wind power receiving unit includes a receiving 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 firstly boosted by the connecting transformer, then passes through the starting circuit, and finally connected to the AC power grid.

[0010] Optionally, the mechanical parts of each wind turbine set are 690V AC, and the electric energy generated by the wind turbine set is rectified into a first direct current of ±550V by the machine-side converter.

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

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

[0013] Optionally, the DC transformer is composed of a plurality of DC converter units, and the plurality of DC converter units are connected in series and parallel;

[0014] The first direct current of ±550V is boosted and rectified by each of the direct current converter units to output rectified direct current;

[0015] The voltage of each DC rectified power is summed to obtain a second DC power of ±30 kV.

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

[0017] In each of the DC transformer units, the first DC power of ±550V is first converted into AC power through the IGBT module, then boosted through the high-frequency transformer, and finally rectified and filtered through the rectifier and filter output to output rectified DC power.

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

[0019] After the ±30kV second DC power is converted by the full-bridge and half-bridge hybrid MMC converter, it is firstly boosted by 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 industrial frequency AC transmission.

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

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

[0022] Optionally, the bipolar DC circuit breakers are respectively a positive DC circuit breaker and a negative DC circuit breaker; the bipolar DC circuit breaker and the full-bridge and half-bridge hybrid MMC converter are both symmetrical unipolar topology structures;

[0023] When the ±30kV DC overhead line fails, 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 respectively perform fault clearing actions to achieve joint coordination of fault clearing of the sending end and the receiving end DC lines; wherein,

[0024] The line protection of the wind power sending end unit generates a tripping instruction, and sends the tripping instruction to the positive DC circuit breaker and the negative DC circuit breaker at the same time 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 instruction, and sends the voltage control instruction to the full-bridge half-bridge hybrid MMC converter, so that the full-bridge half-bridge hybrid MMC converter controls the positive and negative pole voltages on the receiving end DC side to below 0.

[0026] Optionally, when the sending end and the receiving end have completed the fault clearing action, after de-ionization for a 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 reclosed at the same time, 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, the bipolar DC circuit breaker and the full-bridge and half-bridge hybrid MMC converter both adopt a true bipolar topology structure;

[0028] When one of the two poles fails, the faulty pole enters a fault ride-through state, and the DC circuit breaker and the full-bridge and half-bridge hybrid MMC converters of the same pole cooperate to perform a DC line fault clearing action; wherein, one of the two poles is a positive pole, or a negative pole;

[0029] The operation of the line on the pole where no fault occurs is not affected.

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

[0031] Provided is an onshore wind power transmission system. The onshore wind power transmission system includes a wind power sending end unit and a wind power receiving end unit; the wind power sending end unit includes a direct current transformer and two or more wind turbines; each wind turbine is provided with a machine-side converter and corresponds to a low-voltage circuit breaker; a direct current circuit breaker is installed on each bipolar of the direct current transformer; for each wind turbine, the electric energy generated by the wind turbine is rectified into a first direct current through the machine-side converter, and connected in parallel to the direct current transformer through the low-voltage circuit breaker to be boosted into a second direct current, which is sent to the wind power receiving end unit through the bipolar direct current circuit breaker of the direct current transformer; the wind power receiving end unit includes a receiving end converter station, a connecting transformer and a starting circuit; after the second direct current is converted by the receiving end converter station, it is boosted twice by the connecting transformer and connected to the alternating current grid through the starting circuit. The wind turbine output is DC, which is boosted to medium and high-level DC voltage and finally sent out through long-distance overhead lines. This can not only reduce the intermediate AC-DC conversion link, effectively solve the traditional AC collection and transmission problems such as harmonic resonance and reactive power transmission, but also reduce the cost of power generation. The full DC transmission method has higher transmission efficiency and reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

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

[0034] Figure 2 It is a schematic diagram of the circuit principle structure of a wind turbine side rectification;

[0035] Figure 3 The figure is a schematic diagram of the circuit principle structure of a DC transformer unit step-up rectification. DETAILED DESCRIPTION

[0036] The embodiment of the present invention provides an onshore wind power transmission system for solving or partially solving the problems of AC collection and transmission such as harmonic resonance and reactive power transmission existing in the prior art.

[0037] In order to make the purpose, features and advantages of the present invention more obvious and easy to understand, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the embodiments described below are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0038] As an example, taking onshore wind power transmission as an actual application scenario, at present, there are two main types of wind power generation systems: AC collection-AC transmission and AC collection-DC transmission. However, large-scale wind power generation bases have long collection distances. If the AC collection-AC transmission method is adopted, problems such as harmonic resonance and reactive power transmission will be prominent. If the AC collection-DC transmission method is adopted, there will be many power conversion links and high system costs.

[0039] In order to effectively solve the current AC collection and transmission problems such as harmonic resonance and reactive power transmission in onshore wind power transmission, and to achieve efficient grid connection, it is necessary to propose a new onshore wind power transmission topology to reduce the overall cost of the onshore wind power transmission system on the basis of stable onshore wind power transmission.

[0040] Therefore, one of the core invention points of the embodiment of the present invention is: based on a new technical route, the DC transmission technology is organically combined with the DC wind turbine, and an onshore wind power full DC transmission system is proposed. The sending end of the onshore wind power transmission system includes multiple wind turbines and machine-side converters. The low-voltage DC output by the DC wind turbine is boosted and collected by the DC converter, and then connected to the flexible DC converter station at the receiving end through a long-distance overhead line equipped with a DC circuit breaker. The flexible DC converter station at the receiving end adopts a full-bridge and half-bridge hybrid MMC (Modular Multilevel Converter) converter with DC fault clearing capability. The wind turbine output is DC, which is boosted to a medium and high-level DC voltage, and finally sent out through a long-distance overhead line. It can not only reduce the intermediate AC-DC conversion link, effectively solve the traditional AC collection and transmission problems such as harmonic resonance and reactive power transmission, but also reduce the cost of power generation. The full DC transmission method has higher transmission efficiency and reliability.

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

[0042] Overall, the onshore wind power transmission system can be mainly divided into a wind power sending unit (corresponding to the sending end) and a wind power receiving unit (corresponding to the receiving end). Among them, the wind power sending unit can mainly include a DC / DC (Direct Current to Direct Current Converter) DC transformer, two or more wind turbines (for the sake of simplicity, Figure 1 Two permanent magnet direct drive generators are used as examples in this paper. Each wind turbine has an organic side converter (for ease of explanation, Figure 1 Each wind turbine corresponds to a low-voltage circuit breaker.

[0043] For each wind turbine, the electric energy generated by the wind turbine is rectified into the first direct current through the machine-side converter, and connected in parallel to the DC transformer through the low-voltage circuit breaker. The first direct current is boosted into the second direct current through the DC transformer, and the second direct current is sent to the wind power receiving unit through the bipolar DC circuit breaker of the DC transformer.

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

[0045] Next, the ±550V first DC power obtained after rectification by two machine-side converters (or multiple machine-side converters in multiple application scenarios) is connected in parallel to the input end of the DC transformer through a low-voltage circuit breaker, which corresponds to the low-voltage side. The low-voltage side of the DC transformer boosts the ±550V first DC power to a ±30kV second DC power, and outputs it through the high-voltage side of the DC transformer (which can also be understood as boosting to ±30kV on the high-voltage side). A DC circuit breaker is installed on each of the two poles (positive and negative) of the output end of the DC transformer (i.e., corresponding to the high-voltage side). Then it is connected to the receiving-end converter station of the wind power receiving unit via a long-distance overhead line.

[0046] from Figure 1 It can be seen that the DC transformer is composed of a plurality of DC converter units (such as module 1 to module 12), and the plurality of DC converter units are connected in series and parallel.

[0047] When boosting, the first DC power of ±550V is firstly boosted and rectified by each DC converter unit to output rectified DC power, and then the voltage of each rectified DC power is summed to obtain the second DC power of ±30kV.

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

[0049] Take the DC transformer with 12 units as an example, Figure 3 It can be seen that each DC transformer unit is mainly composed 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 first DC power is first converted into AC power through the IGBT module, then boosted by a high-frequency transformer, and finally rectified and filtered by the rectifier and filter output to output rectified DC power.

[0051] More specifically, the DC transformer unit input is ±550V DC, and the input end is composed of an IGBT module. The ±550V first DC is first inverted into a high-frequency low-voltage AC by the IGBT module, and then boosted to a high-frequency high-voltage AC by a high-frequency transformer, and then output through a rectifier and filter (diode rectifier), and after rectification and filtering, it forms a ±2.5kV DC.

[0052] In the embodiment of the present invention, 12 DC transformer units are provided, and each DC transformer unit can obtain a boosted DC power of ±2.5kV. The 12 DC / DC converter units are connected in series and parallel to finally output a second DC power of ±30kV.

[0053] It is understandable that the number of DC transformer units is not necessarily 12, as long as the final output DC voltage is ±30kV. 12 is used here because the high-voltage side output of a single DC transformer unit is ±2.5kV, which is accumulated to ±30kV. For example, ±550V DC, assuming that 3kV DC can be output after being processed by a single DC converter unit, in this case, the DC transformer only needs 10 DC converter units to obtain a total DC power of ±30kV. It is understandable that the present invention is not limited to this.

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

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

[0056] Among them, MMC consists of multiple sub-modules (SM). Each sub-module usually 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 the A, B, and C phases in a three-phase AC system.

[0057] The full-bridge half-bridge hybrid MMC converter provided in the embodiment of the present invention is mainly composed of two submodules: HBSM (Half-Bridge Submodule) and FBSM (Full-Bridge Submodule). Among them, HBSM is composed of two IGBTs and an energy storage capacitor. FBSM is composed of four IGBTs and an energy storage capacitor.

[0058] In other words, the basic topology of the full-bridge and half-bridge hybrid MMC converter at the receiving end can mainly include two sub-modules: full-bridge sub-module and half-bridge sub-module. Therefore, when a fault occurs, the full-bridge sub-module can output a negative level and cooperate with the half-bridge sub-module to control the DC side voltage below 0. The topological structure of the full-bridge and half-bridge hybrid MMC converter partially adopts the half-bridge sub-module. Compared with the full-bridge sub-module, it can reduce the number of power devices and reduce the investment cost of the equipment while realizing the DC side fault ride-through.

[0059] The DC side bipolar of the full-bridge and half-bridge hybrid MMC converter is connected to the DC breaker at the sending end, receiving the ±30kV second DC power, and the AC side is connected to the connection transformer and the starting circuit, and then connected to the large power grid through 35kV industrial frequency AC transmission. That is, after the second DC power is converted by the receiving end converter station, it is first boosted by the connection transformer, then passes through the starting circuit, and finally connected to the AC grid.

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

[0061] The Start-Up Circuit enables the system to start smoothly and reliably after power-on or reset.

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

[0063] The bipolar DC circuit breaker at the sending end is connected to the two ends of the full-bridge and half-bridge hybrid MMC converter at the receiving end through the ±30kV DC overhead line; when a fault occurs in the ±30kV DC overhead line, the full-bridge and half-bridge hybrid MMC converter cooperate with the bipolar DC circuit breaker to clear the DC line fault.

[0064] In combination with the above content, it can be known that the bipolar DC circuit breakers referred to in the present invention are respectively the positive DC circuit breakers and the negative DC circuit breakers. As an optional embodiment, the bipolar DC circuit breakers and the full-bridge and half-bridge hybrid MMC converters are as follows Figure 1 Symmetrical unipolar topology shown.

[0065] Symmetric Unipolar Topology is a topology used in power electronic converters, especially in multi-level converters (such as the full-bridge half-bridge hybrid MMC converter in the invention). This topology is characterized by an equal number of submodules in each phase arm and the same voltage level for each submodule.

[0066] When a ±30kV DC overhead line fails, the line protection of the wind power sending unit and the line protection of the wind power receiving unit detect the fault and perform fault clearing actions respectively to achieve joint coordination of fault clearing of the sending and receiving DC lines.

[0067] Specifically, the line protection of the wind power sending end unit generates a tripping instruction, and sends the tripping instruction to the positive DC circuit breaker and the negative DC circuit breaker at the same time to control the tripping of the positive DC circuit breaker and the negative DC circuit breaker.

[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 half-bridge hybrid MMC converter, so that the full-bridge half-bridge hybrid MMC converter controls the positive and negative voltages on the receiving DC side to below 0.

[0069] Furthermore, when the sending end and the receiving end have completed the fault clearing action, after a preset recovery time (such as hundreds of milliseconds) of de-ionization, the full-bridge and half-bridge hybrid MMC converters restore 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 reclosed at the same time, and the onshore wind power transmission system resumes operation.

[0070] In the embodiment of the present invention, the symmetrical unipolar topology is adopted at the transmitting and receiving ends because the capacity of the system itself is small and the cost of adopting the symmetrical unipolar is low. When the capacity of the system is large, if the symmetrical unipolar topology is continued to be adopted, once a DC line fault occurs, it may have a greater impact on the system. At this time, a true bipolar solution can be considered. In this case, if a fault occurs in one pole, only the pole will enter the fault ride-through state, and only the DC circuit breaker of the pole needs to be tripped, and only the MMC control voltage of the pole needs to be lowered to below 0, and the operation of the other pole will not be affected.

[0071] Then in a specific implementation, when the system capacity is greater than or equal to a preset capacity threshold, both the bipolar DC circuit breaker and the full-bridge and half-bridge hybrid MMC converter adopt a true bipolar topology structure;

[0072] When one of the two poles fails, the faulty pole enters a fault ride-through state, and the DC circuit breaker and the full-bridge and half-bridge hybrid MMC converters of the same pole cooperate to perform the DC line fault clearing action; wherein, one of the two poles is a positive pole, or a negative pole;

[0073] The operation of the line on the pole where no fault occurs is not affected.

[0074] For example, when the system capacity is large, both the bipolar DC circuit breaker and the full-bridge and half-bridge hybrid MMC converter adopt a true bipolar topology. When a fault occurs in the positive pole of the bipolar, the positive pole enters the fault ride-through state, and the DC circuit breaker (corresponding to the positive DC circuit breaker) of the same pole (i.e., both are positive poles) as the fault (i.e., the full-bridge and half-bridge hybrid MMC converters (the end of the full-bridge and half-bridge hybrid MMC converters connected to the positive pole) cooperates to perform the DC line fault clearing action. The line operation of the negative pole where no fault occurs is not affected.

[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, some technical features are distinguished by the first and the second in the embodiments of the present invention. The first and the second are only used for data distinction and have no other special meanings. It can be understood that the present invention is not limited to this.

[0076] In the embodiment of the present invention, based on a brand-new technical route, the DC transmission technology is organically combined with the DC wind turbine, and an onshore wind power full DC transmission system is proposed. The sending end of the onshore wind power transmission system includes multiple wind turbines and machine-side converters. The low-voltage DC output by the DC wind turbine is boosted and collected by the DC converter, and then connected to the flexible DC converter station at the receiving end through a long-distance overhead line equipped with a DC circuit breaker. The flexible DC converter station at the receiving end adopts a full-bridge and half-bridge hybrid MMC converter with DC fault clearing capability. The wind turbine output is DC, which is boosted to a medium- and high-level DC voltage by DC, and finally sent out through a long-distance overhead line. It can not only reduce the intermediate AC-DC conversion link, effectively solve the traditional AC collection and transmission problems such as harmonic resonance and reactive power transmission, but also reduce the cost of power generation. The full DC transmission method has higher transmission efficiency and reliability.

[0077] In the embodiments provided by the present invention, it should be understood that the disclosed system can be implemented in other ways. For example, the device embodiments described above are only schematic, for example, the division of the units is only a logical function division, and there may be other division methods in actual implementation, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point, the mutual coupling or direct coupling or communication connection shown or discussed can be through some interfaces, indirect coupling or communication connection of devices or units, which can be electrical, mechanical or other forms.

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

[0079] In addition, each functional unit in each embodiment of the present invention may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit. The above-mentioned integrated unit may be implemented in the form of hardware or in the form of software functional units.

[0080] If the integrated unit is implemented in the form of 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. The computer software product is stored in a storage medium, including several instructions for a computer device (which can be a personal computer, a server, or a network device, etc.) to perform all or part of the steps of the method described in each embodiment of the present invention. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (ROM), random access memory (RAM), disk or optical disk, etc., various media that can store program codes.

[0081] As described above, the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features thereof may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions 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 comprises a wind power sending end unit and a wind power receiving end unit; wherein, The wind power sending end unit includes a DC transformer and two or more wind turbines; each wind turbine is provided with an organic side converter, and each wind turbine corresponds to a low voltage circuit breaker; a DC circuit breaker is installed at each bipolar output end of the DC transformer; For each wind turbine generator set, the electric energy generated by the wind turbine generator set is rectified into a first direct current through the machine-side converter, and connected in parallel to the direct current transformer through the low-voltage circuit breaker; The first direct current is boosted into a second direct current by the direct current transformer, and the second direct current is sent to the wind power receiving unit through the bipolar direct current circuit breaker of the direct current transformer; The wind power receiving unit includes a receiving converter station, a connecting transformer and a starting circuit; After the second DC power is converted by the receiving-end converter station, it is firstly boosted by the connecting transformer, then passes through the starting circuit, and finally connected to the AC power grid.

2. The onshore wind power transmission system according to claim 1, characterized in that: The mechanical part of each wind turbine generator set is 690V AC, and the electric energy generated by the wind turbine generator set is rectified into a first direct current of ±550V by the machine-side converter.

3. The onshore wind power transmission system according to claim 2, characterized in that: The first direct current of ±550V obtained after rectification by the machine-side converter is connected in parallel to the input end of the direct current transformer through the low-voltage circuit breaker; the input end of the direct current transformer corresponds to the low-voltage side, and the output end corresponds to the high-voltage side; The low-voltage side of the DC transformer boosts the first DC power of ±550V to the second DC power 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 a plurality of DC converter units, and the plurality of DC converter units are connected in series and parallel; The first direct current of ±550V is boosted and rectified by each of the direct current converter units to output rectified direct current; The voltage of each DC rectified power is summed to obtain a second DC power of ±30 kV.

5. The onshore wind power transmission system according to claim 4, characterized in that: Each of the DC transformer units is composed of an IGBT module, a high-frequency transformer, and a rectifier filter output; In each of the DC transformer units, the first DC power of ±550V is first converted into AC power through the IGBT module, then boosted through the high-frequency transformer, and finally rectified and filtered through 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 receiving-end converter station adopts a full-bridge and half-bridge hybrid MMC converter with DC fault clearing capability; After the ±30kV second DC power is converted by the full-bridge and half-bridge hybrid MMC converter, it is firstly boosted by 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 industrial frequency AC transmission.

7. The onshore wind power transmission system according to claim 6, characterized in that: The bipolar DC circuit breaker at the sending end is respectively connected to both ends of the full-bridge and half-bridge hybrid MMC converter at the receiving end through a ±30kV DC overhead line; When the ±30kV DC overhead line fails, the full-bridge and half-bridge hybrid MMC converter cooperates with the bipolar DC circuit breaker to clear the DC line fault.

8. The onshore wind power transmission system according to claim 7, characterized in that: The bipolar DC circuit breakers are respectively a positive DC circuit breaker and a negative DC circuit breaker; the bipolar DC circuit breaker and the full-bridge and half-bridge hybrid MMC converter are both symmetrical unipolar topology structures; When the ±30kV DC overhead line fails, 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 respectively perform fault clearing actions to achieve joint coordination of fault clearing of the sending end and the receiving end DC lines; wherein, The line protection of the wind power sending end unit generates a tripping instruction, and sends the tripping instruction to the positive DC circuit breaker and the negative DC circuit breaker at the same time to control the tripping of the positive DC circuit breaker and the negative DC circuit breaker; The line protection of the wind power receiving unit generates a voltage control instruction, and sends the voltage control instruction to the full-bridge half-bridge hybrid MMC converter, so that the full-bridge half-bridge hybrid MMC converter controls the positive and negative pole voltages on the receiving end DC side to below 0.

9. The onshore wind power transmission system according to claim 8, characterized in that: When the sending end and the receiving end have completed the fault clearing action, after 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 reclosed at the same time, and the onshore wind power transmission system resumes operation.

10. The onshore wind power transmission system according to claim 7, characterized in that: When the system capacity is greater than or equal to a preset capacity threshold, the bipolar DC circuit breaker and the full-bridge and half-bridge hybrid MMC converter both adopt a true bipolar topology structure; When one of the two poles fails, the faulty pole enters a fault ride-through state, and the DC circuit breaker and the full-bridge and half-bridge hybrid MMC converters of the same pole cooperate to perform a DC line fault clearing action; wherein, one of the two poles is a positive pole, or a negative pole; The operation of the line on the pole where no fault occurs is not affected.

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