A decentralized uncontrolled rectifier hybrid DC unidirectional power transmission system and its control method

Abstract

The present invention relates to a distributed uncontrolled rectifier hybrid DC unidirectional power transmission system and its control method. An offshore station includes several wind turbines, several uncontrolled rectifier modules, and a full-bridge MCC. The wind turbines are divided into 2m groups, each group including n wind turbines. The n wind turbines are connected to an uncontrolled rectifier module, for a total of 2m uncontrolled rectifier modules. m of the uncontrolled rectifier modules are connected in series and connected to the positive electrode, and m of the uncontrolled rectifier modules are connected to the negative electrode. Each uncontrolled rectifier module is connected to the full-bridge MMC via a first AC circuit breaker, where n and m are both positive integers. The positive and negative electrodes are respectively connected to the input ends of corresponding DC cables, and the output ends of the DC cables are connected to an onshore station. The onshore station includes a thyristor commutation module connected to the DC cable and a receiving AC power grid connected to the thyristor commutation module. The system has high technical maturity, high system operational reliability, and high economic efficiency. It does not require the configuration of a large number of capacitors and IGBTs.

Description

Technical Field

[0001] The present invention relates to a distributed uncontrolled rectifier hybrid direct current unidirectional power transmission system and a control method thereof, belonging to the technical field of hybrid direct current power transmission. Background Art

[0002] Due to the early and relatively comprehensive development of offshore wind farm resources, offshore wind power development has become the mainstream trend in wind power development. There are two methods for offshore wind power grid connection with engineering experience: high-voltage AC transmission and high-voltage DC transmission. High-voltage AC transmission is widely used in offshore wind power grid connection projects, characterized by mature technology and low cost. However, due to the issue of cable capacitance charging current, the economic transmission distance is generally within 50-80km, making it unsuitable for deep-sea wind power grid connection. Therefore, high-voltage DC transmission is almost the only viable transmission solution for deep-sea grid connection. It has the advantages of large transmission capacity and long transmission distance, and can provide black start power and grid connection power for offshore wind farms.

[0003] Current HVDC transmission solutions generally utilize flexible HVDC transmission systems based on modular multilevel converters (MMCs). However, each converter requires a large number of cascaded submodules, each of which includes several IGBTs and dry-type capacitors. This results in high costs, bulk, and excessive weight. Consequently, offshore converter platforms must be significantly larger, reducing the economic viability of the entire offshore wind power flexible HVDC transmission system. Current technology upgrades are focused on lightweight, compact, and cost-effective solutions, optimized grid connection, and superior control.

[0004] To achieve the above goals, improved DC transmission technology based on diode valves has gradually gained popularity. Its advantages are that diode valves do not require expensive IGBTs and dry capacitors, occupy a compact space, and reduce the cost of offshore platforms. However, this solution still has two problems: 1) how to achieve black start; 2) after a large number of wind turbines are connected, coordinated control between wind turbines is difficult. There are two solutions in the existing technology. One is to use short-distance AC auxiliary submarine cables to provide grid-connected power for offshore platforms, and the other is to add modular multi-level converter auxiliary power supplies between parallel and DC terminals. The problem with the former solution is that it is not suitable for long-distance offshore wind power grid connection, and the problem with the second solution is that the auxiliary power control is relatively complex, and it is difficult to coordinate a large number of wind turbines directly feeding into the diode valve. Summary of the Invention

[0005] In response to the above problems, the purpose of the present invention is to provide a distributed uncontrolled rectifier hybrid DC unidirectional transmission system and its control method, which has high technical maturity, high system operation reliability and high economy. Compared with conventional flexible DC power transmission, there is no need to configure a large number of sub-module capacitors and IGBTs, which can realize the compactness of offshore platforms and effectively reduce system investment costs and operating losses.

[0006] To achieve the above objectives, the present invention proposes the following technical solution: a distributed uncontrolled rectifier hybrid DC unidirectional power transmission system, comprising: an offshore station, a DC cable, and an onshore station; the offshore station comprises a plurality of wind turbines, a plurality of uncontrolled rectifier modules, and a full-bridge MCC; the plurality of wind turbines are divided into 2m groups, each group comprising n wind turbines, and the n wind turbines are connected to an uncontrolled rectifier module, for a total of 2m uncontrolled rectifier modules, wherein m uncontrolled rectifier modules are connected in series and connected to the positive electrode, and another m uncontrolled rectifier modules are connected in series and connected to the negative electrode, and each uncontrolled rectifier module is connected to the full-bridge MMC via a first AC circuit breaker, wherein n and m are both positive integers; the positive electrode and the negative electrode are respectively connected to the input end of the corresponding DC cable, and the output end of the DC cable is connected to the onshore station; the onshore station comprises a thyristor commutation module connected to the DC cable, and a receiving-end AC power grid connected to the thyristor commutation module.

[0007] Furthermore, the uncontrolled rectifier module includes a twelve-pulsating diode valve commutation unit and a first transformer. The twelve-pulsating diode valve commutation unit is connected to the output end of the first transformer, and the input end of the first transformer is connected to a wind farm consisting of n wind turbines through an AC submarine cable.

[0008] Furthermore, the wind turbine includes fan blades, a direct-drive permanent magnet synchronous unit, an AD converter, a DA converter and a second transformer connected in sequence. A capacitor connected in parallel with the AD converter is arranged between the AD converter and the DA converter. The second transformer is connected to the input end of the AC submarine cable through a second AC circuit breaker.

[0009] Furthermore, the full-bridge MCC includes three bridge arms, each bridge arm includes an upper bridge arm and a lower bridge arm, each upper bridge arm and lower bridge arm is provided with N commutation modules, and the N commutation modules are connected in series.

[0010] Furthermore, the input end of the converter module is connected to the first bridge arm, and the output end is connected to the second bridge arm. The first bridge arm and the second bridge arm are connected in parallel. The first bridge arm and the second bridge arm both include an upper bridge arm and a lower bridge arm. A submodule is provided on each of the upper bridge arm and the lower bridge arm. A capacitor connected in parallel with the first bridge arm and the second bridge arm is provided between the two bridge arms.

[0011] Furthermore, the submodule includes an IGBT and a diode connected in reverse parallel thereto, and the commutation module further includes a fast bypass switch connecting the input end and the output end.

[0012] Furthermore, a bypass switch directly connected to the two uncontrolled rectifier modules is provided on the DC side where the full-bridge MCC and the diode-valve commutation unit are connected in parallel on the DC side.

[0013] Furthermore, the thyristor commutation module includes a twelve-pulse thyristor commutation unit and a first transformer, the input end of the twelve-pulse thyristor commutation unit is connected to a DC cable, and the output end thereof is connected to a three-phase transformer, and the output end of the three-phase transformer is connected to a receiving-end AC power grid; an AC filter is provided on the valve side or the grid side of the twelve-pulse thyristor commutation unit, and the AC filter is a single-tuned filter, a double-tuned filter or a triple-tuned filter.

[0014] The present invention also discloses a control method for a distributed uncontrolled rectifier hybrid DC unidirectional power transmission system, which adopts any of the above-mentioned distributed uncontrolled rectifier hybrid DC unidirectional power transmission systems, including: the DC voltage at the DC side outlet of the DC unidirectional power transmission system offshore station is ±U dc(rec) , the power of each uncontrolled rectifier module is controlled according to the average distribution, then the DC voltage U of each uncontrolled rectifier module is rdc For: U rdc =U dc(rec) / m; control the output power of each fan so that the AC voltage U of the rectifier module is not controlled ac for: Control the trigger angle α of the thyristor commutation module, and then control the DC voltage U of the land station dc(inv) Satisfy the following formula constraints:

[0015]

[0016] Among them, P dc is the unipolar transmission power, I dc Delivers current to a single pole.

[0017] Furthermore, when the wind turbine connected to the uncontrolled rectifier module stops operating due to a fault or maintenance, the second AC circuit breaker between the wind turbine and the uncontrolled rectifier module is first disconnected, and then the trigger angle α of the thyristor commutation module is adjusted to reduce the DC voltage of the onshore station so that the DC current at this time is equal to the DC current before the fault or maintenance.

[0018] Furthermore, the specific steps of the control method during the black start of the transmission system are as follows: controlling the output voltage of the DC side of the thyristor commutation module to be a negative voltage, the charging current direction to be a normal transmission direction, and the charging current not exceeding the tolerance of the equipment; when the voltage of the capacitor in the commutation module reaches a preset value, triggering the capacitor in the commutation module to charge in an orderly manner; after charging is completed, closing the first AC circuit breaker one by one, charging and starting the wind turbines connected to the uncontrolled rectifier module, and then disconnecting the second AC circuit breaker until all wind turbines are started and the DC system enters normal operation.

[0019] The present invention has the following advantages due to the adoption of the above technical solution:

[0020] 1. The present invention adopts a distributed diode uncontrolled rectifier module to access the wind farm at the offshore station and a thyristor commutation module at the onshore station. The technology is highly mature, the system operation reliability is high, and the economy is high. Compared with conventional flexible direct current power transmission, there is no need to configure a large number of sub-module capacitors and IGBTs, which can realize the compactness of the offshore platform and effectively reduce the system investment cost and operating loss.

[0021] 2. The offshore station of the present invention is equipped with a full-bridge modular multi-level converter connected in series with a diode valve, which is connected in series with the DC side of the diode valve. During the black start process, it can provide black start power for the wind turbine; during normal operation, it can serve as dynamic reactive power supplement and active filter to improve the grid-connected performance of the wind turbine.

[0022] 3. The number of wind turbines connected to each diode valve commutation unit of the offshore station of the present invention is relatively small, and compared with the conventional direct feeding of dozens or even hundreds of wind turbines, the control and coordination complexity is greatly reduced.

[0023] 4. The full-bridge modular multi-level converter of the offshore station of the present invention is equipped with a bypass switch. During normal operation, the bypass switch bypasses the converter to reduce power operation loss. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 1. It is a structural diagram of a distributed uncontrolled rectifier hybrid DC unidirectional power transmission system according to an embodiment of the present invention;

[0025] Figure 2 is a schematic diagram of an uncontrolled rectifier module in one embodiment of the present invention;

[0026] Figure 3 is a schematic diagram of a fan circuit in one embodiment of the present invention;

[0027] Figure 4 is a schematic diagram of a full-bridge MMC according to an embodiment of the present invention;

[0028] Figure 5 Schematic diagram of a commutation module in a full-bridge MMC according to an embodiment of the present invention;

[0029] Figure 6 is a schematic diagram of a thyristor commutation module according to an embodiment of the present invention;

[0030] Figure 7 is a schematic diagram of a single tuned filter according to an embodiment of the present invention;

[0031] Figure 8 is a schematic diagram of a double-tuned filter according to an embodiment of the present invention;

[0032] Figure 9 FIG. 1 is a schematic diagram of a triple tuned filter according to an embodiment of the present invention. DETAILED DESCRIPTION

[0033] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the present invention will be described in detail through specific embodiments. However, it should be understood that the specific embodiments are provided only for a better understanding of the present invention and should not be construed as limiting the present invention. In the description of the present invention, it should be understood that the terms used are for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0034] In order to solve the problems existing in the prior art, such as the transmission system is not suitable for long-distance offshore wind power grid connection, or the auxiliary power supply control is relatively complex, and the coordination of a large number of wind turbines directly feeding into the diode valve is relatively difficult. The present invention proposes a distributed uncontrolled rectifier hybrid DC unidirectional transmission system and its control method. The sending end divides the wind farm into slices, distributes and connects several uncontrolled rectifier modules of the offshore station on the AC side, collects energy in a centralized manner on the DC side, and sends it to the onshore station through a submarine cable. During normal operation, the power direction flows from the offshore station to the onshore station; the full-bridge MMC provides black start power for the uncontrolled rectifier module. Each uncontrolled rectifier unit is directly connected to the wind turbines in each zone after zoning, reducing the difficulty of coordinated control of the wind turbines. It can be widely used in the field of flexible DC transmission technology. The scheme of the present invention is described in detail below with reference to several embodiments in conjunction with the accompanying drawings.

[0035] Example 1

[0036] Figure 1 FIG. 1 is a structural diagram of a distributed uncontrolled rectifier hybrid DC unidirectional power transmission system according to an embodiment of the present invention. Figure 1 As shown, the distributed uncontrolled rectifier hybrid DC unidirectional power transmission system in this embodiment includes: an offshore station, a DC cable and an onshore station.

[0037] The offshore station includes several wind turbines, several uncontrolled rectifier modules, and a full-bridge modular multilevel converter (MCC). The wind turbines are divided into 2m groups, each containing n wind turbines. Each n wind turbine is connected to an uncontrolled rectifier module, for a total of 2m uncontrolled rectifier modules. m of the uncontrolled rectifier modules are connected in series and connected to the positive electrode, and m of the uncontrolled rectifier modules are connected in series and connected to the negative electrode. Both n and m are positive integers, and the positive and negative electrodes here refer to the positive and negative electrodes of the entire offshore station circuit, not the positive and negative electrodes of a specific module or unit. Each uncontrolled rectifier module is connected to the full-bridge MMC via a first AC circuit breaker. The full-bridge MCC is connected in parallel with the diode valve commutation unit on the DC side, positioned between the positive and negative distributed diode valves. A bypass switch is provided on the DC side of the full-bridge MMC, directly connected to the two uncontrolled rectifier modules.

[0038] The structure of the uncontrolled rectifier module is as follows Figure 2As shown, it includes a twelve-pulsating diode valve commutation unit and a first transformer. In this embodiment, the first transformer is a three-phase three-winding transformer. The twelve-pulsating diode valve commutation unit is connected to the output end of the three-phase three-winding transformer. The input end of the first transformer is connected to n wind turbines via an AC submarine cable. Figure 2 As shown, in a three-phase, three-winding transformer, signals are input from the medium-voltage winding and output from the high-voltage and low-voltage windings, respectively. Each phase of the high-voltage and low-voltage windings is connected to a bridge arm, meaning that each high-voltage winding and low-voltage winding is connected to three bridge arms. Each bridge arm includes an upper arm and a lower arm, each of which is equipped with a diode. The upper arm of the corresponding high-voltage winding arm is connected to the high-voltage terminal, while its lower arm is connected to the upper arm of the corresponding low-voltage winding arm. The lower arm of the corresponding low-voltage winding arm is connected to the low-voltage terminal.

[0039] like Figure 3 As shown, in this embodiment, the wind turbines are direct-drive wind turbines. Each wind turbine includes a fan blade, a direct-drive permanent magnet synchronous unit, an AD converter, a DA converter, and a first transformer, which are connected in sequence. A capacitor connected in parallel with the AD converter is provided between the AD converter and the DA converter. The first transformer is connected to the input end of the AC submarine cable via a second AC circuit breaker. The output end of the AC submarine cable is connected to the input end of the first transformer.

[0040] like Figure 4 As shown, the full-bridge MCC includes three bridge arms, each of which includes an upper bridge arm and a lower bridge arm. Each upper bridge arm and lower bridge arm is provided with N commutation modules, and the N commutation modules are connected in series. Figure 5 As shown, the input of the commutation module is connected to the first bridge arm, and the output is connected to the second bridge arm. The first and second bridge arms are connected in parallel. The first and second bridge arms each include an upper bridge arm and a lower bridge arm, each with a submodule. A capacitor is provided between the first and second bridge arms in parallel. The submodule includes an IGBT and a diode connected in antiparallel therewith. The commutation module also includes a fast bypass switch connecting the input and output terminals.

[0041] The positive and negative poles are connected to the input ends of corresponding DC cables. This means there are two DC cables: one connected to the positive pole and the other to the negative pole. The output end of the DC cable is connected to an onshore station. The onshore station includes a thyristor commutation module connected to the DC cable and a receiving AC grid connected to the thyristor commutation module.

[0042] like Figure 6As shown, the thyristor commutation module includes a twelve-pulse thyristor commutation unit and a third transformer. The input of the twelve-pulse thyristor commutation unit is connected to a DC cable, and its output is connected to a three-phase transformer. The output of the three-phase transformer is connected to the receiving AC grid. In this embodiment, the third transformer is also a three-phase, three-winding transformer. The structure of the thyristor commutation module is similar to that of the uncontrolled rectifier module, except that signals are input from the high-voltage winding and low-voltage winding, respectively, and output from the medium-voltage winding. Thyristors are installed in each upper and lower bridge arm instead of diodes.

[0043] An AC filter is installed on the valve side or grid side of the twelve-pulse thyristor commutation unit. The valve side refers to the area between the bridge arm and the third transformer, while the grid side refers to the area between the third transformer and the receiving AC grid. The decision on whether to place the filter on the valve side or the grid side requires a comprehensive consideration of engineering, technical, and economic considerations. The AC filter can be either active or passive. If passive, it is preferably a single-tuned filter, a double-tuned filter, or a triple-tuned filter.

[0044] The structure of a single tuned filter is as follows Figure 7 As shown in Figure 1, it includes capacitors and inductors connected in series. The structure of the double-tuned filter is shown in Figure 1. Figure 8 As shown, the triple-tuned filter includes a first capacitor, a first inductor, and a second capacitor connected in series, a first resistor connected in parallel with the first inductor and the second capacitor, a second resistor connected in series with the second inductor and in parallel with the first inductor and the second capacitor, and a third resistor connected in parallel with the second inductor. Figure 9 As shown, it includes a first capacitor, a second capacitor and a first inductor connected in series, and a first resistor, a second capacitor and the first inductor connected in parallel.

[0045] Example 2

[0046] Based on the same inventive concept, this embodiment discloses a control method for a distributed uncontrolled rectifier hybrid DC unidirectional power transmission system, using a distributed uncontrolled rectifier hybrid DC unidirectional power transmission system as any of the above. During normal operation, in this embodiment, the wind turbines are grid-controlled to control the AC voltage and frequency within a specified range. The number n of wind turbines generally does not exceed 10 to reduce the difficulty of coordinated control between wind turbines.

[0047] The control method includes:

[0048] The DC voltage at the DC side outlet of the DC one-way transmission system offshore station is ±U dc(rec) , the power of each uncontrolled rectifier module is controlled according to the average distribution, then the output power P of each uncontrolled rectifier module is r For: P r =P dc / m, where m is the number of uncontrolled rectifier modules set at the positive or negative pole, Pdc It delivers power to the single pole.

[0049] Each uncontrolled rectifier module DC voltage U rdc For: U rdc =U dc(rec) / m;

[0050] Output power P of each fan w For: P w =P r / n;

[0051] Control the output power of each fan so that the AC voltage U ac for:

[0052] Control the trigger angle α of the thyristor commutation module, and then control the DC voltage U of the land station dc(inv) Satisfy the following formula constraints:

[0053]

[0054] Among them, I dc Delivers current to a single pole.

[0055] When the wind turbine connected to the uncontrolled rectifier module stops operating due to a fault or maintenance, the second AC circuit breaker between the wind turbine and the uncontrolled rectifier module is first disconnected. At this time, the DC side voltage of the uncontrolled rectifier unit is zero. Then, the trigger angle α of the thyristor commutation module is adjusted to reduce the DC voltage of the onshore station so that the DC current at this time is equivalent to the DC current before the fault or maintenance.

[0056] The specific steps of the control method during black start of the transmission system are as follows:

[0057] Connect all relevant switches in the entire transmission system and unlock the thyristor commutation module of the onshore station;

[0058] Control the DC side output voltage of the thyristor commutation module to be negative, the charging current direction is the normal power transmission direction, and the charging current does not exceed the tolerance of the equipment;

[0059] When the voltage of the capacitor in the commutation module reaches a preset value, the capacitor in the commutation module is triggered to charge in an orderly manner;

[0060] After charging is completed, the first AC circuit breaker is closed one by one to charge and start the fans connected to the uncontrolled rectifier modules, and then the second AC circuit breaker is disconnected until all fans are started and the DC system enters normal operation.

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the specific embodiments of the present invention can still be modified or replaced by equivalents, and any modifications or equivalent replacements that do not depart from the spirit and scope of the present invention should be included within the scope of protection of the claims of the present invention. The above content is only a specific embodiment of the present application, but the scope of protection of the present application is not limited thereto. Any person skilled in the art who is familiar with the technical field can easily think of changes or replacements within the technical scope disclosed in the present application, which should be included within the scope of protection of the present application. Therefore, the scope of protection of the present application should be based on the scope of protection of the claims.

Claims

1. A control method for a distributed uncontrolled rectifier hybrid DC unidirectional power transmission system, using the following distributed uncontrolled rectifier hybrid DC unidirectional power transmission system, comprising: Offshore stations, DC cables and onshore stations; The offshore station includes a plurality of wind turbines, a plurality of uncontrolled rectifier modules and a full-bridge MCC. The plurality of wind turbines are divided into 2m groups, each group includes n wind turbines, and the n wind turbines are connected to an uncontrolled rectifier module, and are connected to a total of 2m uncontrolled rectifier modules, wherein m uncontrolled rectifier modules are connected in series and connected to the positive electrode of the offshore station, and another m uncontrolled rectifier modules are connected in series and connected to the negative electrode of the offshore station, and each uncontrolled rectifier module is connected to the full-bridge MMC via a first AC circuit breaker, wherein n and m are both positive integers; The positive electrode and the negative electrode are respectively connected to the input end of the corresponding DC cable, and the output end of the DC cable is connected to the land station; The onshore station includes a thyristor commutation module connected to the DC cable, and a receiving-end AC power grid connected to the thyristor commutation module; It is characterized in that it includes the following steps: The DC voltage at the DC side outlet of the DC one-way transmission system offshore station is ± U dc(rec) , the power of each uncontrolled rectifier module is controlled according to the average distribution, then the DC voltage of each uncontrolled rectifier module is U rdc for: Wherein, m is the number of uncontrolled rectifier modules; According to the DC voltage, the output power of each fan is adjusted so that the AC voltage of the uncontrolled rectifier module U ac for: ； According to the AC voltage output by the uncontrolled rectifier module of the offshore station, the trigger angle α of the thyristor commutation module of the onshore station is controlled so that the DC voltage of the onshore station U dc(inv) Satisfies the following formula: in, P dc For unipolar power transmission, I dc Delivers current to a single pole.

2. The control method of the distributed uncontrolled rectifier hybrid DC unidirectional power transmission system according to claim 1, characterized in that: The uncontrolled rectifier module includes a twelve-pulsating diode valve commutation unit and a first transformer. The twelve-pulsating diode valve commutation unit is connected to the output end of the first transformer, and the input end of the first transformer is connected to n wind turbines through an AC submarine cable.

3. The control method of the distributed uncontrolled rectifier hybrid DC unidirectional power transmission system according to claim 2, characterized in that: The wind turbine includes fan blades, a direct-drive permanent magnet synchronous unit, an AD converter, a DA converter and a second transformer connected in sequence. A capacitor connected in parallel with the AD converter is arranged between the AD converter and the DA converter. The second transformer is connected to the input end of the AC submarine cable through a second AC circuit breaker.

4. The control method of the distributed uncontrolled rectifier hybrid DC unidirectional power transmission system according to claim 1, characterized in that: The full-bridge MCC includes three bridge arms, each bridge arm includes an upper bridge arm and a lower bridge arm, each upper bridge arm and lower bridge arm are provided with N full-bridge MCC commutation modules, and the N full-bridge MCC commutation modules are connected in series.

5. The control method of the distributed uncontrolled rectifier hybrid DC unidirectional power transmission system according to claim 4, characterized in that: The commutation module in the full-bridge MCC includes a first bridge arm, a second bridge arm, a submodule, and a capacitor. The input end of the commutation module in the full-bridge MCC is connected to the first bridge arm, and the output end is connected to the second bridge arm. The first bridge arm and the second bridge arm are connected in parallel. The first bridge arm and the second bridge arm each include an upper bridge arm and a lower bridge arm. A submodule is respectively provided on the upper bridge arm and the lower bridge arm. A capacitor is provided between the first bridge arm and the second bridge arm and connected in parallel with the first bridge arm and the second bridge arm.

6. The control method of the distributed uncontrolled rectifier hybrid DC unidirectional power transmission system according to claim 4 or 5, characterized in that: The full-bridge MCC and the diode valve commutation unit are connected in parallel on the DC side. A bypass switch directly connected to two uncontrolled rectifier modules is provided on the DC side of the full-bridge MCC.

7. The control method of the distributed uncontrolled rectifier hybrid DC unidirectional power transmission system according to claim 1, characterized in that: The thyristor commutation module includes a twelve-pulse thyristor commutation unit and a first transformer. The input end of the twelve-pulse thyristor commutation unit is connected to the DC cable, and the output end thereof is connected to a three-phase transformer. The output end of the three-phase transformer is connected to the receiving-end AC power grid. An AC filter is provided on the valve side or the grid side of the twelve-pulse thyristor commutation unit. The AC filter is a single-tuned filter, a double-tuned filter, or a triple-tuned filter.

8. The control method of the distributed uncontrolled rectifier hybrid DC unidirectional power transmission system according to claim 1, characterized in that: When the wind turbine connected to the uncontrolled rectifier module stops operating due to a fault or maintenance, the second AC circuit breaker between the wind turbine and the uncontrolled rectifier module is first disconnected, and then the trigger angle α of the thyristor commutation module is adjusted to reduce the DC voltage of the onshore station so that the DC current of the onshore station at this time is equal to the DC current before the fault or maintenance.

9. The control method of the distributed uncontrolled rectifier hybrid DC unidirectional power transmission system according to claim 1, characterized in that: The control method during the black start of the power transmission system is as follows: the DC side output voltage of the thyristor commutation module is controlled to be a negative voltage, and the charging current direction is the normal power transmission direction; when the voltage of the capacitor of the commutation module in the full-bridge MCC reaches a preset value, the capacitor of the commutation module in the full-bridge MCC is triggered to charge; after charging is completed, the first AC circuit breaker is closed, the wind turbine connected to the uncontrolled rectifier module is charged and started, and then the second AC circuit breaker is disconnected until all wind turbines are started and normal operation is entered.

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