New energy transmission system through hybrid direct current transmission

Abstract

The invention relates to the technical field of new energy, and discloses a system for sending out new energy through hybrid direct current transmission. The system comprises a receiving-end power grid, a receiving-end converter station, a sending-end converter station and a sending-end new energy source. The sending end converter station comprises a first diode valve, a sending end flexible direct current converter valve, a second diode valve, a filter and a sending end alternating current bus; the first diode valve, the sending end flexible direct current converter valve, the second diode valve and the filter are respectively connected with the sending end alternating current bus; wherein the first diode valve is connected with a sending end alternating current bus through a first transformer and a second transformer which are connected in parallel; the second diode valve is connected with the sending end AC bus through a third transformer and a fourth transformer which are connected in parallel; the first diode valve is connected with the receiving end converter station through the positive power transmission line, and the second diode valve is connected with the receiving end converter station through the negative power transmission line. According to the invention, the power supply reliability of hybrid direct current transmission is improved on the premise of not reducing the electric energy quality of the alternating current side.

Description

Technical Field

[0001] This invention relates to the field of new energy technology, and specifically to a new energy transmission system via hybrid DC power transmission. Background Technology

[0002] Currently, most operational offshore wind power systems utilize flexible DC transmission, but the size, weight, and cost of offshore flexible DC converter platforms are excessive. To achieve compactness and lightweight design of offshore converter platforms, reduce the cost of offshore converter valves, and ensure compatibility with both grid-connected and grid-connected wind turbines, engineers are exploring hybrid DC transmission technology combining diodes and MMC (Multi-Channel Modulated Controller) for offshore wind power. In this technology, both the diode valves connected to the positive and negative transmission lines at offshore converter stations are 12-pulse diode valves. A problem arises with the valve-side winding connection of the connected transformers: if a transformer on one pole fails, the entire system shuts down, or the corresponding normally operating transformer on the other pole is taken out of service. In this case, the entire system requires a DC voltage reduction to operate, and this also places additional requirements on the onshore converter station for DC voltage reduction. Therefore, this transformer valve-side winding connection not only reduces power supply reliability but also significantly increases the cost of equipment at the receiving-end converter station. Summary of the Invention

[0003] In view of this, the present invention provides a new energy transmission system via hybrid DC transmission to solve the problem of improving the power supply reliability of hybrid DC transmission in the event of a transformer failure on one side.

[0004] This invention provides a new energy source transmission system via hybrid DC transmission, comprising: a receiving-end power grid, a receiving-end converter station, a sending-end converter station, and a sending-end new energy source; the receiving-end power grid is connected to the receiving-end converter station; the receiving-end converter station is connected to the sending-end converter station; and the sending-end converter station is connected to the sending-end new energy source.

[0005] The sending-end converter station includes a first diode valve, a sending-end flexible DC converter valve, a second diode valve, a filter, and a sending-end AC bus. The first diode valve, the sending-end flexible DC converter valve, the second diode valve, and the filter are respectively connected to the sending-end AC bus. Among them, the first diode valve is connected to the sending-end AC bus through a first transformer and a second transformer connected in parallel; the second diode valve is connected to the sending-end AC bus through a third transformer and a fourth transformer connected in parallel; the first diode valve is connected to the receiving-end converter station through a positive transmission line, and the second diode valve is connected to the receiving-end converter station through a negative transmission line.

[0006] With the system provided in this embodiment, if a transformer on a certain pole fails, it is only necessary to stop the operation of the failed transformer, without stopping the entire system or taking the normally operating transformers on the other pole's transmission line out of operation. At the same time, this system still has the ability to reduce the AC side harmonics to only 12k±1 harmonics, reducing the risk of overall system shutdown due to transformer failure, ensuring AC side power quality, and maintaining the system's stable and efficient operation.

[0007] In one optional implementation, the valve-side winding connection method of the first transformer is the same as that of the second transformer, the valve-side winding connection method of the third transformer is the same as that of the fourth transformer, and the valve-side winding connection method of the first transformer is different from that of the third transformer.

[0008] Through the above implementation method, the first and third transformers adopt different valve-side winding connection methods. For example, one transformer uses a star connection and the other uses a delta connection, with a phase angle difference of 30° between the two transformers. This achieves effective control and optimization of AC side harmonics, ensuring that the AC side harmonics only exhibit the 12k±1st harmonic, thus guaranteeing power quality. This differentiated wiring allows for minimizing the impact of a transformer failure on the overall system operation, maintaining continuous power supply, and continuously leveraging the harmonic suppression advantage through specific wiring combinations. This ensures stable and efficient system operation, greatly improving the system's reliability and adaptability.

[0009] In one alternative implementation, the valve-side winding is connected in a star configuration or a delta configuration.

[0010] In one alternative implementation, the first diode valve, together with the first transformer and the second transformer, constitutes a 6-pulse diode valve, and the second diode valve, together with the third transformer and the fourth transformer, constitutes a 6-pulse diode valve.

[0011] In the embodiments of this application, by using a specific wiring method with the transformer, it is still possible to achieve a performance with only 12k±1 harmonics on the AC side, thereby further improving the reliability and fault response capability of the system while ensuring power quality.

[0012] In one alternative implementation, the target transformer is taken out of operation in the event of a fault, while other transformers, except the target transformer, operate normally. The target transformer is any one of the first transformer, the second transformer, the third transformer, and the fourth transformer.

[0013] Through the above implementation method, when the target transformer fails, only that transformer needs to be disconnected, while the remaining transformers can continue to operate normally. This avoids the entire system from shutting down due to the failure of a single device, greatly improving the continuity and reliability of the system's power supply and reducing economic losses caused by power outages. This mechanism also reduces the number of start-ups and shutdowns of non-faulty equipment, reducing equipment wear and tear, extending their service life, and lowering operation and maintenance costs. Simultaneously, this flexible fault handling method, combined with specific transformer valve-side winding wiring and a 6-pulse diode valve system design, ensures that the disconnection of a single transformer does not affect the overall operation, while still maintaining AC side harmonics with only 12k±1 harmonics. This ensures that power quality is not affected, guarantees stable and efficient system operation, and significantly enhances the overall system's resilience and adaptability.

[0014] In one alternative implementation, the filter is used to filter out the AC side 12k±1st harmonic of the AC bus voltage output at the sending end, where k is a positive integer.

[0015] Through the above implementation methods, the use of filters to remove 12k±1 harmonics can significantly improve the power quality on the AC side, reduce harmonic interference to the power grid and electrical equipment, and ensure the safe and stable operation of various electrical equipment. When combined with the system's 6-pulse diode valve and specific transformer wiring methods, the filter can continue to suppress harmonics even when a single transformer fails and is disconnected, ensuring that the system meets power quality requirements under different operating conditions. Furthermore, the targeted design of this filter, which is solely for removing 12k±1 harmonics on the AC side, reduces unnecessary harmonic filtering losses, improves system energy transmission efficiency, and simultaneously reduces equipment capacity requirements and filter equipment costs, achieving a dual optimization of system economy and reliability.

[0016] In one alternative implementation, the first diode valve, the feed-end flexible DC converter valve, and the second diode valve are connected in series on the DC side, or the first diode valve, the feed-end flexible DC converter valve, and the second diode valve are connected in parallel on the DC side.

[0017] Through the above implementation methods, whether the first diode valve, the sending-end flexible DC converter valve, and the second diode valve are connected in series on the DC side, or connected in parallel on the DC side, both methods are compatible with the above-mentioned transformer winding side wiring, 6-pulse diode valve, and other designs. When a single transformer fails and is taken out of service, the remaining equipment will operate normally. In this way, the scope of the fault impact is reduced, the harmonic characteristics of the AC side are not affected, the power quality stability of the system is maintained, and the continuous operation of the system is guaranteed.

[0018] In one alternative implementation, the sending-end converter station further includes a fifth transformer, through which the sending-end flexible DC converter valve is connected to the sending-end AC bus.

[0019] In one alternative implementation, the receiving-end converter station includes a sixth transformer and a receiving-end flexible DC converter valve, the receiving-end flexible DC converter valve being connected to the receiving-end power grid via the sixth transformer. Attached Figure Description

[0020] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of a new energy transmission system via hybrid DC power transmission according to an embodiment of the present invention. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] This application provides a new energy transmission system via hybrid DC power transmission, which aims to improve the power supply reliability of hybrid DC power transmission in the event of a transformer failure on one side.

[0024] Figure 1 This is a schematic diagram of a new energy transmission system via hybrid DC power transmission according to an embodiment of the present invention, as shown below. Figure 1 As shown, the system includes: receiving-end power grid 1, receiving-end converter station 2, sending-end converter station 3, and sending-end renewable energy 4; receiving-end power grid 1 is connected to receiving-end converter station 2; receiving-end converter station 2 is connected to sending-end converter station 3; sending-end converter station 3 is connected to sending-end renewable energy 4.

[0025] Specifically, receiving-end grid 1 refers to the regional power grid that receives and consumes electricity transmitted over long distances. Receiving-end converter station 2, located on the receiving-end grid 1 side, is responsible for converting high-voltage direct current (HVDC) power into alternating current (AC) power and connecting it to receiving-end grid 1. Sending-end converter station 3, located on the sending-end renewable energy side, converts AC power into DC power and transmits it to receiving-end grid 1 via transmission lines. Sending-end renewable energy 4 refers to renewable energy power generation equipment located near sending-end converter station 3, which can be wind farms, photovoltaic power plants, etc., serving as the source of electricity transmission.

[0026] The sending-end converter station 3 includes a first diode valve 31, a sending-end flexible DC converter valve 32, a second diode valve 33, a filter 34, and a sending-end AC bus 35. The first diode valve 31, the sending-end flexible DC converter valve 32, the second diode valve 33, and the filter 34 are all connected to the sending-end AC bus 35. The first diode valve 31 is connected to the sending-end AC bus 35 through a first transformer 36 and a second transformer 37 connected in parallel. The second diode valve 33 is connected to the sending-end AC bus 35 through a third transformer 38 and a fourth transformer 39 connected in parallel. The first diode valve 31 is connected to the receiving-end converter station 2 through a positive transmission line, and the second diode valve 33 is connected to the receiving-end converter station 2 through a negative transmission line.

[0027] Specifically, the first diode valve 31 and the second diode valve 33 are rectifier / inverter devices composed of multiple diodes connected in series or parallel. The sending-end flexible DC converter valve 32 is a converter valve based on voltage source converter technology. It adopts a half-bridge sub-module structure and is composed of insulated gate bipolar transistors (IGBTs), anti-parallel diodes, and capacitors. It can realize flexible DC transmission and has bidirectional control capability for reactive and active power.

[0028] Filter 34 is a device used to filter out the 12k±1st harmonics (such as the 11th and 13th harmonics) on the AC side and improve power quality. It is usually composed of capacitors, inductors and resistors to form an inductor-capacitor (LC) filter circuit.

[0029] The sending-end AC bus 35 is used to collect AC power output from the sending-end new energy source 4 within the converter station.

[0030] The first transformer 36, the second transformer 37, the third transformer 38, and the fourth transformer 39 are devices used to change voltage levels. In this embodiment, a 12-pulse rectification effect is achieved through a preset wiring method.

[0031] Taking the sending-end new energy 4 as an example of an offshore wind turbine, in the system provided in this application embodiment, the offshore wind turbine transmits electrical energy to the offshore converter station (i.e., the sending-end AC bus 35) via the offshore AC bus. The flexible DC converter valve in the offshore converter station converts the AC power into DC power. At the same time, the first diode valve 31 and the second diode valve 33 assist in rectification or fault isolation. The filter 34 filters out harmonics at the output of the sending-end AC bus 35 to ensure that the power quality meets the standards. Finally, the offshore converter station transmits the DC power to the onshore converter station (i.e., the receiving-end converter station 2) through the positive and negative transmission lines. The onshore converter station converts the DC power into AC power to connect to the receiving-end power grid 1.

[0032] With the system provided in this embodiment, if a transformer on a certain pole fails, it is only necessary to stop the operation of the failed transformer, without stopping the entire system or taking the normally operating transformers on the other pole's transmission line out of operation. At the same time, this system still has the ability to reduce the AC side harmonics to only 12k±1 harmonics, reducing the risk of overall system shutdown due to transformer failure, ensuring AC side power quality, and maintaining the system's stable and efficient operation.

[0033] In some embodiments, the valve-side winding connection method of the first transformer 36 is the same as that of the valve-side winding connection method of the second transformer 37, the valve-side winding connection method of the third transformer 38 is the same as that of the valve-side winding connection method of the fourth transformer 39, and the valve-side winding connection method of the first transformer 36 is different from that of the third transformer 38.

[0034] Specifically, a transformer is a device that uses the principle of electromagnetic induction to change AC voltage. In the sending-end converter station 3, it is used to realize voltage level transformation, electrical isolation, and to cooperate with the converter valve to complete the power conversion. In the embodiment of this application, four transformers are connected in pairs to the first diode valve 31 and the second diode valve 33 respectively, to assist in realizing the conversion of power from AC to DC.

[0035] The winding connecting the transformer to the diode valve is called the valve-side winding. Its voltage level and wiring method must match the operating characteristics of the diode valve to ensure efficient energy conversion and transmission. The wiring methods for the valve-side winding between the transformer and the diode valve include, but are not limited to, star (Y) and delta (Δ) connections. Different wiring methods affect the voltage and current transformation relationships and harmonic characteristics of the transformer. In this embodiment, the wiring methods of the valve-side windings of various transformers are combined to achieve harmonic suppression.

[0036] When the first transformer 36 and the second transformer 37 are connected in parallel, their valve-side windings use the same connection method, such as a star (Y) connection. This identical connection method ensures consistency in their operating characteristics, facilitating the maintenance of the same harmonic suppression effect through the parallel transformer in the event of a single transformer failure. Similarly, when the third transformer 38 and the fourth transformer 39 are connected in parallel, their valve-side windings also use the same connection method, such as a delta (Δ) connection. This group of transformers also exhibits similar operating characteristics due to the identical connection method, ensuring the maintenance of the same harmonic suppression effect through the parallel transformer in the event of a single transformer failure.

[0037] In this embodiment, the first transformer 36 and the third transformer 38 employ different valve-side winding connection methods. For example, one transformer uses a star connection, while the other uses a delta connection, with a 30° phase angle difference between the two transformers. This effectively controls and optimizes AC side harmonics, ensuring that only the 12k±1st harmonic is present, thus guaranteeing power quality. This differentiated wiring allows for minimizing the impact of a transformer failure on the overall system operation, maintaining continuous power supply, and leveraging the specific combination of wiring methods to continuously exert harmonic suppression advantages, ensuring stable and efficient system operation and significantly improving system reliability and adaptability.

[0038] In some embodiments, the valve-side winding is connected in a star configuration or a delta configuration.

[0039] In some embodiments, the first diode valve 31, together with the first transformer 36 and the second transformer 37, constitutes a 6-pulse diode valve, and the second diode valve 33, together with the third transformer 38 and the fourth transformer 39, constitutes a 6-pulse diode valve.

[0040] Specifically, a star (Y) connection refers to connecting the three-phase ends of the transformer valve-side winding to a common point (neutral point), with the three phase start-up points led out separately to form a "Y"-shaped connection. With this connection method, the neutral point can be grounded or ungrounded, allowing for flexible adjustment of the system voltage level and neutral point operation mode.

[0041] A delta (Δ) connection refers to connecting the three phases of the transformer valve-side winding sequentially (e.g., the end of phase A is connected to the beginning of phase B, the end of phase B is connected to the beginning of phase C, and the end of phase C is connected to the beginning of phase A) to form a closed delta (Δ) connection. In this connection method, the winding line voltage is equal to the phase voltage, and there is no neutral point.

[0042] Taking the valve-side winding connection of the first transformer 36 and the second transformer 37 as examples (both are star-connected), and the valve-side winding connection of the third transformer 38 and the fourth transformer 39 as examples (both are delta-connected), the first transformer 36 is connected to a 6-pulse diode valve (first diode valve 31), and the third transformer 38 is connected to another 6-pulse diode valve (second diode valve 33). The DC voltages output by these two transformers are superimposed to form a 12-pulse rectified waveform. This is because the output voltages of the star-connected first transformer 36 and the delta-connected third transformer 38 have a 30° phase difference. Superposition of these voltages eliminates the 5th and 7th harmonics, making the AC harmonics mainly the 11th and 13th harmonics (i.e., 12k ± 1, when k = 1), thereby reducing harmonic pollution to the power grid and improving power quality. Similarly, the DC voltages output by the second transformer 37 and the fourth transformer 39 can also form a 12-pulse rectified waveform when superimposed.

[0043] In the embodiments of this application, by using a specific wiring method with the transformer, it is still possible to achieve a performance with only 12k±1 harmonics on the AC side, thereby further improving the reliability and fault response capability of the system while ensuring power quality.

[0044] In some embodiments, the target transformer is taken out of operation in the event of a fault, while other transformers besides the target transformer operate normally, wherein the target transformer is any one of the first transformer 36, the second transformer 37, the third transformer 38, and the fourth transformer 39.

[0045] Specifically, the target transformer refers to the transformer in the sending-end converter station 3 that has experienced a fault. The fault can be a winding short circuit, insulation damage, overload burnout, etc. In the event of a fault in a transformer in the sending-end converter station 3, the faulty transformer can be isolated by protective devices (such as circuit breakers, fuses, etc.) to prevent the fault from spreading, while ensuring that other transformers in normal condition continue to operate, thus maintaining the reliable operation of the sending-end converter station 3.

[0046] For example, a relay protection device can be used to detect whether a transformer has failed and trigger the high-voltage side and valve side circuit breakers of the corresponding transformer to trip, thus disconnecting the faulty transformer from the system.

[0047] For example, in the event of a fault in the first transformer 36 (star-connected Y), the second transformer 37 (star-connected Y), the third transformer 38 (delta-connected Δ), and the fourth transformer 39 (delta-connected Δ) can continue to operate. Simultaneously, the second transformer 37, the third transformer 38, and the fourth transformer 39 result in only 12k±1 harmonics on the AC side. This is because the output voltage of the second transformer 37 (star-connected) maintains a 30° phase difference with the output voltage of the third / fourth transformer 39 (delta-connected). The second transformer 37 (star-connected Y) and the delta transformer group (third transformer 38 + fourth transformer 39 in parallel) are each connected to a 6-pulse diode valve. The superposition of their DC voltages still forms a 12-pulse waveform, thus the AC side harmonics remain at 12k±1. Similarly, in the event of a fault in the third transformer 38, the first transformer 36, the second transformer 37, and the fourth transformer 39 can continue to operate, and simultaneously, the first transformer 36, the second transformer 37, and the fourth transformer 39 result in only 12k±1 harmonics on the AC side. This is because the output voltage of the first transformer 36 / second transformer 37 (star-connected in parallel) and the output voltage of the fourth transformer 39 (delta-connected) still maintain a 30° phase difference. The star-connected transformer group (first transformer 36 + second transformer 37 in parallel) and the delta-connected transformer (fourth transformer 39) are each connected to a 6-pulse diode valve. The superposition of their DC voltages still results in 12 pulses, and the AC side harmonic characteristics remain unchanged. In other words, as long as both Y and Δ connections exist in the remaining transformers, regardless of their quantity (≥1 unit), the 30° phase difference remains unchanged. Therefore, the 12-pulse rectification effect is maintained.

[0048] Through the embodiments of this application, when the target transformer fails, only that transformer needs to be disconnected, while the remaining transformers can continue to operate normally. This avoids the entire system from shutting down due to the failure of a single device, greatly improving the continuity and reliability of the system's power supply and reducing economic losses caused by power outages. This mechanism also reduces the number of start-ups and shutdowns of non-faulty equipment, reducing equipment wear and tear, extending their service life, and lowering maintenance costs. Simultaneously, this flexible fault handling method, combined with specific transformer valve-side winding wiring and a 6-pulse diode valve system design, ensures that the disconnection of a single transformer does not affect the overall operation, while still maintaining AC side harmonics with only 12k±1 harmonics. This ensures that power quality is not affected, guarantees stable and efficient system operation, and significantly enhances the overall system's resilience and adaptability.

[0049] In some embodiments, filter 34 is used to filter out the AC side 12k±1st harmonic of the voltage output of the sending AC bus 35, where k is a positive integer.

[0050] For example, filter 34 includes, but is not limited to, components such as inductors and capacitors. For instance, filter 34 employs an LC filter circuit (inductor and capacitor connected in series or parallel). By adjusting the parameters of the inductor and capacitor, the circuit presents low impedance to specific frequencies (such as the 11th and 13th harmonics), thereby guiding harmonic current into filter 34 and preventing it from being injected into the AC bus. For the 12k±1st harmonic, multiple LC branches can be used to resonate at the 11th, 13th, and 23rd harmonic frequencies, respectively, forming a multi-tuned filter 34.

[0051] Through the embodiments of this application, the use of filter 34 to filter out the 12k±1st harmonic can significantly improve the power quality on the AC side, reduce the interference of harmonics on the power grid and electrical equipment, and ensure the safe and stable operation of various electrical equipment. When filter 34 is combined with the 6-pulse diode valve and the specific wiring method of the transformer in the system, it can still play a harmonic suppression role when a single transformer fails and is out of service, ensuring that the system meets the power quality requirements under different operating conditions. In addition, the targeted design of filter 34, which is only used to filter out the 12k±1st harmonic on the AC side, can reduce unnecessary harmonic filtering losses, improve the system energy transmission efficiency, reduce equipment capacity requirements, reduce the equipment cost of filter 34, and achieve dual optimization of system economy and reliability.

[0052] In some embodiments, the first diode valve 31, the feed-end flexible DC converter valve 32, and the second diode valve 33 are connected in series on the DC side, or the first diode valve 31, the feed-end flexible DC converter valve 32, and the second diode valve 33 are connected in parallel on the DC side.

[0053] Of course, the first diode valve 31, the sending-end flexible DC converter valve 32, and the second diode valve 33 can also be connected in series and parallel or in a bridge configuration on the DC side. This application does not make specific limitations on this, as long as the first diode valve 31, the sending-end flexible DC converter valve 32, and the second diode valve 33 are connected in parallel on the AC side.

[0054] Through the embodiments of this application, whether the first diode valve 31, the sending-end flexible DC converter valve 32, and the second diode valve 33 are connected in series on the DC side or in parallel on the DC side, both methods are compatible with the above-mentioned transformer winding side wiring, 6-pulse diode valve, and other designs. When a single transformer fails and is taken out of service, the remaining equipment will operate normally. In this way, the scope of fault impact is reduced, the harmonic characteristics of the AC side are not affected, the power quality stability of the system is maintained, and the continuous operation of the system is guaranteed.

[0055] In some embodiments, the sending-end converter station 3 further includes a fifth transformer, and the sending-end flexible DC converter valve 32 is connected to the sending-end AC bus 35 through the fifth transformer.

[0056] Specifically, the fifth transformer can be used to adjust voltage levels, frequency, etc., to achieve matching between the sending-end AC bus 35 and the sending-end flexible DC converter valve 32.

[0057] In some embodiments, the receiving-end converter station 2 includes a sixth transformer and a receiving-end flexible DC converter valve, the receiving-end flexible DC converter valve being connected to the receiving-end power grid 1 through the sixth transformer.

[0058] For example, the AC voltage output from the receiving-end flexible DC converter valve needs to be stepped down (or stepped up) by a sixth transformer to the rated voltage of the receiving-end power grid 1 (e.g., from high voltage to medium voltage distribution network voltage) to achieve safe grid connection. Furthermore, the sixth transformer can also be used to isolate the converter valve from faults in the receiving-end power grid 1; for example, in the event of a short circuit in the converter valve, the impedance of the sixth transformer can limit the fault current.

[0059] Although embodiments of the invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the invention, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A new energy source transmission system via hybrid DC power transmission, characterized in that, The system includes: a receiving-end power grid, a receiving-end converter station, a sending-end converter station, and a sending-end renewable energy source; the receiving-end power grid is connected to the receiving-end converter station; the receiving-end converter station is connected to the sending-end converter station; and the sending-end converter station is connected to the sending-end renewable energy source. The sending-end converter station includes a first diode valve, a sending-end flexible DC converter valve, a second diode valve, a filter, and a sending-end AC bus. The first diode valve, the sending-end flexible DC converter valve, the second diode valve, and the filter are respectively connected to the sending-end AC bus. The first diode valve is connected to the sending-end AC bus through a first transformer and a second transformer connected in parallel. The second diode valve is connected to the sending-end AC bus through a third transformer and a fourth transformer connected in parallel. The first diode valve is connected to the receiving-end converter station through a positive transmission line, and the second diode valve is connected to the receiving-end converter station through a negative transmission line.

2. The system according to claim 1, characterized in that, The valve-side winding connection method of the first transformer is the same as that of the second transformer, the valve-side winding connection method of the third transformer is the same as that of the fourth transformer, and the valve-side winding connection method of the first transformer is different from that of the third transformer.

3. The system according to claim 2, characterized in that, The valve-side winding is connected in either a star configuration or a delta configuration.

4. The system according to any one of claims 1-3, characterized in that, The first diode valve, together with the first transformer and the second transformer, constitutes a 6-pulse diode valve. The second diode valve, together with the third transformer and the fourth transformer, constitutes a 6-pulse diode valve.

5. The system according to claim 4, characterized in that, If the target transformer fails, it will be taken out of operation, while other transformers will operate normally. The target transformer is any one of the first transformer, the second transformer, the third transformer, and the fourth transformer.

6. The system according to claim 4, characterized in that, The filter is used to filter out the 12k±1st harmonic of the AC side of the AC bus voltage output at the sending end, where k is a positive integer.

7. The system according to claim 6, characterized in that, The first diode valve, the feed-end flexible DC converter valve, and the second diode valve are connected in series on the DC side, or the first diode valve, the feed-end flexible DC converter valve, and the second diode valve are connected in parallel on the DC side.

8. The system according to any one of claims 1-3, characterized in that, The sending-end converter station also includes a fifth transformer, and the sending-end flexible DC converter valve is connected to the sending-end AC bus through the fifth transformer.

9. The system according to any one of claims 1-3, characterized in that, The receiving-end converter station includes a sixth transformer and a receiving-end flexible DC converter valve, which is connected to the receiving-end power grid through the sixth transformer.

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