A new energy through true bipolar hybrid direct current transmission system

Abstract

The application relates to the new energy technology field and discloses a new energy true bipolar hybrid DC power transmission system. 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 receiving end converter station is connected with the sending end converter station in a bipolar metal middle line mode; in the receiving end converter station, a first HVDC valve and a second HVDC valve are connected in series at a DC side, and are connected with a receiving end AC bus at an AC side respectively; in the sending end converter station, each diode valve, a third HVDC valve and a fourth HVDC valve are connected in series at the DC side, and are connected with a sending end AC bus at the AC side through a transformer respectively. Through the application, the system adopts a true bipolar main wiring mode, realizes reverse power transmission from the receiving end converter station to the sending end converter station, and reduces the equipment cost for realizing black start of the sending end converter station.

Description

Technical Field

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

[0002] Currently, offshore wind power mainly utilizes flexible DC transmission technology for power transmission and grid connection. However, the large size and weight of the offshore converter platforms under this technology severely restrict the large-scale cluster development of offshore wind power. To achieve lightweight offshore converter platforms, engineers in related fields are exploring offshore wind power DC transmission technology based on a hybrid of diodes and flexible DC. However, due to the unidirectional conduction characteristics of diodes, it is difficult to provide black-start power to offshore wind farms through onshore reverse power transmission as in pure flexible DC transmission. In addition, existing offshore wind power DC transmissions all adopt a symmetrical monopolar topology. As offshore wind power capacity continues to grow, to further improve power supply reliability, its topology will inevitably develop towards true bipolar in the future. Therefore, how to combine hybrid DC transmission with a true bipolar topology to solve the black-start problem at low cost while ensuring power supply reliability is a current focus. Summary of the Invention

[0003] In view of this, the present invention provides a new energy true bipolar hybrid DC transmission system to solve the problem of reducing the cost of the sending-end converter station while realizing the reverse power transmission from the receiving-end converter station to the sending-end converter station.

[0004] The system includes: Receiving-end power grid, receiving-end converter station, sending-end converter station, and sending-end renewable energy; 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 via a bipolar band metal neutral line; the sending-end converter station is connected to the sending-end renewable energy. The receiving-end converter station includes a first flexible DC converter valve, a second flexible DC converter valve, and a receiving-end AC bus; the first flexible DC converter valve and the second flexible DC converter valve are connected in series on the DC side, and the first flexible DC converter valve and the second flexible DC converter valve are respectively connected to the receiving-end AC bus on the AC side. The sending-end converter station includes a first diode valve, a second diode valve, a third flexible DC converter valve, a fourth flexible DC converter valve, a third diode valve, a fourth diode valve, and a sending-end AC bus. The first diode valve, the second diode valve, the third flexible DC converter valve, the fourth flexible DC converter valve, the third diode valve, and the fourth diode valve are respectively connected to the sending-end AC bus on the AC side. The sending-end AC bus is connected to the sending-end renewable energy source. The first diode valve, the second diode valve, the third flexible DC converter valve, the fourth flexible DC converter valve, the third diode valve, and the fourth diode valve are connected in series on the DC side; the first diode valve is connected to the receiving-end converter station through the positive pole transmission line, and the third flexible DC converter valve is connected to the positive pole transmission line by a first bypass switch; the fourth diode valve is connected to the receiving-end converter station through the negative pole transmission line; the third flexible DC converter valve and the fourth flexible DC converter valve are connected to the first flexible DC converter valve and the second flexible DC converter valve through a metal return line.

[0005] The system provided in this embodiment configures the flexible DC transmission system as a hybrid flexible DC system combining a true bipolar sending-end flexible DC converter valve and a diode valve. Both the first and second flexible DC converter valves can feed power back to the sending-end flexible DC converter valve (such as the third and fourth flexible DC converter valves) via the transmission line, overcoming the limitation of the unidirectional conduction of diode valves which cannot feed power back. This allows the receiving-end converter station to feed power back to the sending-end converter station for black start, eliminating the need for additional starting equipment at the sending-end converter station and reducing equipment costs. Furthermore, the receiving-end and sending-end converter stations are connected by a metallic return line, ensuring that the system can retain power transmission capability on the other pole even in the event of a fault on one pole, thus achieving fault isolation and continuous partial power transmission. For example, when the positive pole transmission line fails, the system can still transmit power through the loop formed by the negative pole line and the metallic return line.

[0006] In one optional embodiment, the receiving-end converter station further includes a fifth flexible DC converter valve and a sixth flexible DC converter valve; the fifth flexible DC converter valve, the first flexible DC converter valve, the second flexible DC converter valve, and the sixth flexible DC converter valve are connected in series on the DC side and connected in parallel on the AC side after passing through their corresponding connecting transformers. The fifth flexible DC converter valve is connected to the positive transmission line, and the sixth flexible DC converter valve is connected to the negative transmission line; a first DC disconnect switch is connected between the first diode valve and the positive transmission line, and a second DC disconnect switch is connected between the fifth flexible DC converter valve and the positive transmission line; a second bypass switch is connected between the first flexible DC converter valve and the positive transmission line.

[0007] Through the above implementation method, a first bypass switch is connected between the third flexible DC converter valve and the positive transmission line, and a second bypass switch is connected between the first flexible DC converter valve and the positive transmission line, which allows for flexible activation and deactivation of different converter valves. For example, during black start-up, by disconnecting the first and second DC disconnect switches and closing the first and second bypass switches, power can be fed back to the third flexible DC converter valve through the first flexible DC converter valve to start the third flexible DC converter valve, and further start the fourth flexible DC converter valve. As another example, after the sending-end renewable energy source starts up, by closing the first and second DC disconnect switches and disconnecting the first and second bypass switches, the electrical energy of the sending-end renewable energy source is transmitted to the receiving-end grid through the first diode valve, second diode valve, third flexible DC converter valve, fourth flexible DC converter valve, third diode valve, fourth diode valve, fifth flexible DC converter valve, first flexible DC converter valve, second flexible DC converter valve, and sixth flexible DC converter valve. In this system, by setting a first bypass switch, a second bypass switch, a first DC disconnect switch, and a second DC disconnect switch, the system can be flexibly switched from black start state to normal operation state, thereby enhancing the system's adaptability to different operating conditions.

[0008] In one alternative embodiment, a third bypass switch is connected between the fourth flexible DC converter valve and the negative transmission line; a fourth bypass switch is connected between the second flexible DC converter valve and the negative transmission line.

[0009] Through the above implementation method, similar to the first and second bypass switches, bypass switches are connected to the fourth and second flexible DC converter valves respectively, which can also flexibly realize the engagement and disengagement of different converter valves. For example, during black start, by flexibly controlling the closing of the bypass switches, the second flexible DC converter valve can be used to charge and unlock the fourth flexible DC converter valve. Furthermore, after the sending-end renewable energy source starts up, disconnecting the third and fourth bypass switches allows the power from the sending-end renewable energy source to be transmitted to the receiving-end grid through the first diode valve, second diode valve, third flexible DC converter valve, fourth flexible DC converter valve, third diode valve, fourth diode valve, fifth flexible DC converter valve, first flexible DC converter valve, second flexible DC converter valve, and sixth flexible DC converter valve.

[0010] In one alternative implementation, a third DC disconnect switch is connected between the fourth diode valve and the negative transmission line, and a fourth DC disconnect switch is connected between the sixth flexible DC converter valve and the negative transmission line.

[0011] Through the above implementation methods, the system can flexibly respond to different operating conditions, such as black start, normal operation, and faults, by setting the third and fourth DC disconnect switches. Taking the failure of the fourth diode valve or the sixth flexible DC converter valve as an example, the corresponding third and fourth DC disconnect switches can be disconnected to quickly isolate the faulty equipment from the negative transmission line, prevent the fault from spreading, reduce the scope of the fault's impact, and improve system reliability. Taking the black start process as an example, after the first flexible DC converter valve has completed charging the third flexible DC converter valve, the second flexible DC converter valve has completed charging the fourth flexible DC converter valve, and the sending-end renewable energy has started, the third and fourth DC disconnect switches are closed, allowing the electrical energy generated by the sending-end renewable energy to be transmitted to the receiving-end grid.

[0012] In one alternative implementation, the rated DC voltage of the first flexible DC converter valve is the same as that of the third flexible DC converter valve.

[0013] Through the above implementation method, during the black-start reverse power transmission process, when the receiving-end converter station transmits power back to the third flexible DC converter valve of the sending-end converter station through the first flexible DC converter valve, the same rated DC voltage value allows the transmitted power to match the third flexible DC converter valve. Due to voltage matching, the third flexible DC converter valve does not require additional voltage regulation equipment to adapt to the input voltage, and can quickly enter the working state, thereby stably starting the fourth flexible DC converter valve and the entire system. This avoids start-up failure, start-up delay, or equipment impact caused by voltage mismatch, significantly improving the reliability of black start, allowing the system to resume operation in the shortest possible time, and reducing the loss of new energy power caused by shutdown.

[0014] In one alternative implementation, the rated DC voltage of the second flexible DC converter valve is the same as that of the fourth flexible DC converter valve.

[0015] Through the above implementation method, similar to how the rated DC voltage values ​​of the first and third flexible DC converter valves are the same, the rated DC voltage values ​​of the second and fourth flexible DC converter valves are the same. This allows the fourth flexible DC converter valve to be started smoothly without an additional voltage regulation device when the receiving-end converter station feeds back power to the sending-end fourth flexible DC converter valve through the second flexible DC converter valve. This avoids startup failures, delays, or equipment damage caused by voltage mismatch, improves black start reliability, shortens system recovery time, and reduces losses from renewable energy power outages.

[0016] In one optional embodiment, the sending-end converter station further includes a first rectifier transformer, a second rectifier transformer, a third rectifier transformer, and a fourth rectifier transformer; a first diode valve is connected to the sending-end AC bus via the first rectifier transformer, a second diode valve is connected to the sending-end AC bus via the second rectifier transformer, a third diode valve is connected to the sending-end AC bus via the third rectifier transformer, and a fourth diode valve is connected to the sending-end AC bus via the fourth rectifier transformer.

[0017] Through the above implementation methods, the voltage level can be flexibly adjusted using a rectifier transformer to achieve voltage matching between each diode valve and the sending-end AC bus. Furthermore, the electrical energy generated by new energy sources may contain harmonics; the rectifier transformer can also act as a filter and isolater, reducing the impact of harmonics on the diode valves and subsequent DC circuits, improving power quality, and enabling more efficient energy transfer from the AC side to the DC side, thus enhancing the overall system's transmission performance. Simultaneously, the rectifier transformer also provides electrical isolation; in the event of a fault in the sending-end new energy source or diode valve, the rectifier transformer can prevent the fault from spreading, preventing the fault range from expanding, and improving the overall system's safety and reliability.

[0018] In one optional embodiment, the receiving-end converter station further includes a first connecting transformer, a second connecting transformer, a third connecting transformer, and a fourth connecting transformer; a fifth flexible DC converter valve is connected to the receiving-end AC bus via the first connecting transformer, a first flexible DC converter valve is connected to the receiving-end AC bus via the second connecting transformer, a second flexible DC converter valve is connected to the receiving-end AC bus via the third connecting transformer, and a sixth flexible DC converter valve is connected to the receiving-end AC bus via the fourth connecting transformer.

[0019] Through the above implementation method, each flexible DC converter valve is connected to the receiving-end power grid via its corresponding connecting transformer, which can accurately match the voltage level of the converter valve and the receiving-end power grid. Different flexible DC converter valves have different output characteristics; the connecting transformer connected to the flexible DC converter valve can uniformly adapt its output voltage to the receiving-end power grid voltage, avoiding connection failures or energy losses caused by voltage mismatch and ensuring a smooth transition of power from the converter valve to the grid. Furthermore, in terms of fault isolation, if a flexible DC converter valve or connecting transformer fails, the faulty part can be isolated by disconnecting the corresponding connecting transformer, while other normal converter valves can still supply power to the grid normally, minimizing the scope of the fault's impact.

[0020] In one optional implementation, the sending-end converter station further includes a fifth connecting transformer and a sixth connecting transformer; the third flexible DC converter valve is connected to the sending-end AC bus via the fifth connecting transformer, and the fourth flexible DC converter valve is connected to the sending-end AC bus via the sixth connecting transformer.

[0021] Through the above implementation method, the fifth and sixth connecting transformers can achieve precise matching of the voltage levels of the third and fourth flexible DC converter valves and the sending-end AC bus. Furthermore, if the third flexible DC converter valve or the fifth connecting transformer fails, the connection between the fifth connecting transformer and the sending-end AC bus or the third flexible DC converter valve can be disconnected, isolating the faulty part. At this time, the fourth flexible DC converter valve can still operate normally through the sixth connecting transformer. Similarly, if the fourth flexible DC converter valve or the sixth connecting transformer fails, the third flexible DC converter valve can continue to operate, reducing the scope of the fault's impact, decreasing the risk of system downtime, improving the reliability of the sending-end converter station, and ensuring the continuous transmission of new energy power. Attached Figure Description

[0022] 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.

[0023] Figure 1 This is a schematic diagram of a new energy true bipolar hybrid DC transmission system provided according to an embodiment of the present invention. Detailed Implementation

[0024] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. 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.

[0025] In the description of this invention, it should be noted that the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, unless otherwise explicitly specified and limited, the terms "installed," "connected," and "linked" should be interpreted broadly. For example, they can refer to fixed connections, mechanical connections, or electrical connections; they can also refer to direct connections or indirect connections through an intermediate medium; they can also refer to the internal connection of two components; and they can be wireless or wired connections. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0026] This invention provides a new energy transmission system via a true bipolar hybrid DC transmission system, such as... Figure 1As shown, the system includes: a receiving-end power grid 1, a receiving-end converter station 2, a sending-end converter station 3, and a sending-end renewable energy source 4. The receiving-end power grid 1 is connected to the receiving-end converter station 2; the receiving-end converter station 2 is connected to the sending-end converter station 3 via a bipolar metal neutral line; the sending-end converter station 3 is connected to the sending-end renewable energy source 4.

[0027] Specifically, the receiving-end converter station 2 is used to convert DC power into AC power for connection to the receiving-end power grid 1.

[0028] The sending-end converter station 3 is a facility used to convert the AC power generated by the sending-end renewable energy 4 into DC power for external transmission, such as an offshore sending-end converter platform. The sending-end converter station 3 can be connected to the receiving-end converter station 2 via positive and negative transmission lines.

[0029] Sending-end new energy 4 refers to new energy power generation facilities located at the power sending end. For example, sending-end new energy 4 can be an offshore wind farm, photovoltaic power station, etc. Sending-end new energy 4 is connected to the sending-end AC bus via transmission cables, and transmits the generated AC power to the sending-end AC bus. After being converted by the sending-end converter station 3, it is transmitted to the receiving-end converter station 2 and the receiving-end power grid 1.

[0030] The receiving-end converter station 2 includes a first flexible DC converter valve 21, a second flexible DC converter valve 22, and a receiving-end AC bus 213. The first flexible DC converter valve 21 and the second flexible DC converter valve 22 are connected in series on the DC side, and the first flexible DC converter valve 21 and the second flexible DC converter valve 22 are respectively connected to the receiving-end AC bus 213 on the AC side.

[0031] Specifically, the flexible DC converter valve is a converter valve based on voltage source converter technology. For example, the flexible DC converter valve in this embodiment can adopt a half-bridge sub-module structure, composed of an insulated gate bipolar transistor (IGBT), anti-parallel diodes, and capacitors, etc., enabling flexible DC transmission and possessing bidirectional control capability for reactive and active power. Of course, the flexible DC converter valve can also adopt a full-bridge sub-module structure. This application does not specifically limit the specific structure of the flexible DC converter valve; it can be set according to actual needs.

[0032] The sending-end converter station 3 includes a first diode valve 31, a second diode valve 32, a third flexible DC converter valve 33, a fourth flexible DC converter valve 34, a third diode valve 35, a fourth diode valve 36, and a sending-end AC bus 37. The first diode valve 31, the second diode valve 32, the third flexible DC converter valve 33, the fourth flexible DC converter valve 34, the third diode valve 35, and the fourth diode valve 36 are respectively connected to the sending-end AC bus 37 on the AC side. The sending-end AC bus 37 is connected to the sending-end new energy 4.

[0033] The first diode valve 31, the second diode valve 32, the third flexible DC converter valve 33, the fourth flexible DC converter valve 34, the third diode valve 35, and the fourth diode valve 36 are connected in series on the DC side; the first diode valve 31 is connected to the receiving-end converter station 2 through the positive pole transmission line, and the third flexible DC converter valve 33 is connected to the positive pole transmission line by a first bypass switch 38; the fourth diode valve 36 is connected to the receiving-end converter station 2 through the negative pole transmission line; the third flexible DC converter valve 33 and the fourth flexible DC converter valve 34 are connected to the first flexible DC converter valve 21 and the second flexible DC converter valve 22 through a metal return line.

[0034] Specifically, the third flexible DC converter valve 33 and the fourth flexible DC converter valve 34 are connected to the first flexible DC converter valve 21 and the second flexible DC converter valve 22 via a metal return line, making the entire system a bipolar wiring configuration with a metal return line. In this system, the positive and negative transmission lines form a bipolar main circuit. The electrical energy generated by the new energy source 4 at the sending end is transmitted through the positive and negative transmission lines, and the unbalanced current flowing through the metal return line is close to zero. The metal return line connects the third and fourth flexible DC converter valves at the sending end to the first and second flexible DC converter valves at the receiving end. When a fault occurs in one pole of the main circuit (such as a fault in the positive transmission line), the current can form a loop through the metal return line, ensuring that the system can continue to operate under single-pole fault conditions, avoiding the shutdown of the entire system and improving the reliability of the system.

[0035] In this embodiment, the flexible DC transmission system is configured as a hybrid flexible DC system combining a true bipolar sending-end flexible DC converter valve and a diode valve. Both the first flexible DC converter valve 21 and the second flexible DC converter valve 22 can feed power back to the sending-end flexible DC converter valve (such as the third flexible DC converter valve 33 and the fourth flexible DC converter valve 34) via the transmission line. This overcomes the limitation of the unidirectional conduction of the diode valve, which prevents back-feeding. This allows the receiving-end converter station 2 to feed power back to the sending-end converter station 3 for black start, eliminating the need for additional starting equipment at the sending-end converter station 3 and reducing its equipment cost. Furthermore, the receiving-end converter station 2 and the sending-end converter station 3 are connected by a metallic return line. Even in the event of a fault on one pole of the system, the ability to transmit power on the other pole can still be maintained, thus achieving fault isolation and continuous transmission of some power. For example, when the positive pole transmission line fails, the system can still transmit power through the loop formed by the negative pole line and the metallic return line, ensuring the stable transmission of new energy power and the continuous power supply to the receiving-end grid 1.

[0036] like Figure 1 As shown, the receiving-end converter station 2 also includes a fifth flexible DC converter valve 23 and a sixth flexible DC converter valve 24; the fifth flexible DC converter valve 23, the first flexible DC converter valve 21, the second flexible DC converter valve 22, and the sixth flexible DC converter valve 24 are connected in series on the DC side and connected in parallel on the AC side after passing through their corresponding connecting transformers.

[0037] Specifically, the fifth flexible DC converter valve 23 and the sixth flexible DC converter valve 24, similar to the first flexible DC converter valve 21 and the second flexible DC converter valve 22, serve as flexible DC converter valves for AC-DC conversion in the receiving-end converter station 2. In this embodiment, the fifth flexible DC converter valve 23 and the sixth flexible DC converter valve 24 are used in series with the first flexible DC converter valve 21 and the second flexible DC converter valve 22 after the third flexible DC converter valve 33 and the fourth flexible DC converter valve 34 have finished charging and unlocked, and after the new energy power supply has started, to further improve the DC side voltage level and meet the requirements of large-capacity power transmission.

[0038] like Figure 1 As shown, the fifth flexible DC converter valve 23 is connected to the positive transmission line, and the sixth flexible DC converter valve 24 is connected to the negative transmission line. A first DC disconnect switch 39 is connected between the first diode valve 31 and the positive transmission line, and a second DC disconnect switch 26 is connected between the fifth flexible DC converter valve 23 and the positive transmission line. A second bypass switch 25 is connected between the first flexible DC converter valve 21 and the positive transmission line.

[0039] Specifically, the first bypass switch 38 is connected between the third flexible DC converter valve 33 and the positive transmission line, and is used to bypass the first diode valve 31 and the second diode valve 32. The second bypass switch 25 is connected between the first flexible DC converter valve 21 and the positive transmission line, and is used to bypass the fifth flexible DC converter valve 23. The first DC disconnect switch 39 is a DC isolation device connected between the first diode valve 31 and the positive transmission line, and the second DC disconnect switch 26 is a DC isolation device located between the fifth flexible DC converter valve 23 and the positive transmission line, used to achieve mechanical isolation.

[0040] The third flexible DC converter valve 33 is connected to the positive power transmission line by a first bypass switch 38, and the first flexible DC converter valve 21 is connected to the positive power transmission line by a second bypass switch 25, which can flexibly realize the entry and exit of different converter valves.

[0041] For example, during a black start, disconnecting the first DC switch 39 and the second DC switch 26, and closing the first bypass switch 38 and the second bypass switch 25, allows reverse power to be supplied to the third flexible DC converter valve 33 through the first flexible DC converter valve 21, thereby starting the third flexible DC converter valve 33 and further starting the fourth flexible DC converter valve 34.

[0042] For example, after the sending-end renewable energy source 4 starts up, the first DC disconnect switch 39 and the second DC disconnect switch 26 are closed, and the first bypass switch 38 and the second bypass switch 25 are opened. This allows the electrical energy from the sending-end renewable energy source 4 to be transmitted to the receiving-end power grid 1 through the first diode valve 31, the second diode valve 32, the third flexible DC converter valve 33, the fourth flexible DC converter valve 34, the third diode valve 35, the fourth diode valve 36, the fifth flexible DC converter valve 23, the first flexible DC converter valve 21, the second flexible DC converter valve 22, and the sixth flexible DC converter valve 24. In this system, by setting the first bypass switch 38, the second bypass switch 25, the first DC disconnect switch 39, and the second DC disconnect switch 26, the system can flexibly switch from the black start state to the normal operation state, enhancing the system's adaptability to different operating conditions.

[0043] exist Figure 1 In the middle, the fourth flexible DC converter valve 34 is connected to the negative pole transmission line by a third bypass switch 310; the second flexible DC converter valve 22 is connected to the negative pole transmission line by a fourth bypass switch 27.

[0044] Similar to the first bypass switch 38 and the second bypass switch 25, bypass switches are connected to the fourth flexible DC converter valve 34 and the second flexible DC converter valve 22 respectively, which can also flexibly realize the engagement and disengagement of different converter valves.

[0045] For example, during a black start, the second flexible DC converter valve 22 is used to charge and unlock the fourth flexible DC converter valve 34 by flexibly controlling the closing of the bypass switch.

[0046] For example, after the sending-end new energy source 4 is started, disconnecting the third bypass switch 310 and the fourth bypass switch 27 allows the electrical energy of the sending-end new energy source 4 to be transmitted to the receiving-end power grid 1 through the first diode valve 31, the second diode valve 32, the third flexible DC converter valve 33, the fourth flexible DC converter valve 34, the third diode valve 35, the fourth diode valve 36, the fifth flexible DC converter valve 23, the first flexible DC converter valve 21, the second flexible DC converter valve 22, and the sixth flexible DC converter valve 24.

[0047] exist Figure 1 In the middle, the fourth diode valve 36 is connected to the negative transmission line by a third DC switch 311, and the sixth flexible DC converter valve 24 is connected to the negative transmission line by a fourth DC switch 28.

[0048] In this way, by setting the third DC disconnect switch 311 and the fourth DC disconnect switch 28, the system can flexibly respond to different operating conditions, such as black start, normal operation, and fault.

[0049] Taking the failure of the fourth diode valve 36 or the sixth flexible DC converter valve 24 as an example, the faulty equipment can be quickly isolated from the negative power transmission line by disconnecting the corresponding third DC switch 311 and fourth DC switch 28, so as to prevent the fault from spreading, reduce the scope of the fault's impact, and improve the system reliability.

[0050] Taking the black start process as an example, after the first flexible DC converter valve 21 completes charging of the third flexible DC converter valve 33 and the second flexible DC converter valve 22 completes charging of the fourth flexible DC converter valve 34, and after the sending-end new energy 4 starts, the third DC disconnect switch 311 and the fourth DC disconnect switch 28 are closed, so that the electrical energy generated by the sending-end new energy 4 is transmitted to the receiving-end power grid 1.

[0051] In this embodiment, the DC voltage level corresponding to the fifth flexible DC converter valve 23 is higher than that corresponding to the first flexible DC converter valve 21. The DC voltage level corresponding to the sixth flexible DC converter valve 24 is higher than that corresponding to the second flexible DC converter valve 22. Alternatively, the fifth flexible DC converter valve 23 and the sixth flexible DC converter valve 24 can be understood as high-flexibility DC converter valves, while the first flexible DC converter valve 21 and the second flexible DC converter valve 22 are low-flexibility DC converter valves. That is, the flexible DC converter valves closer to the metallic return line are low-flexibility DC converter valves, and the flexible DC converter valves closer to the positive and negative transmission lines are high-flexibility DC converter valves. Both high-flexibility DC converter valves and low-flexibility DC converter valves are used for AC / DC conversion during normal system operation. The high-flexibility DC converter valves, being closer to the positive and negative transmission lines, can withstand higher DC voltages, while the low-flexibility DC converter valves, being closer to the metallic return line, have lower operating voltages and can function during system black start-up, thus enhancing the flexibility of system operation.

[0052] exist Figure 1 In this process, the rated DC voltage of the first flexible DC converter valve 21 is the same as the rated DC voltage of the third flexible DC converter valve 33.

[0053] Thus, during the black-start reverse power transmission process, when the receiving-end converter station 2 transmits power back to the sending-end converter station 3's third flexible DC converter valve 33 through the first flexible DC converter valve 21, the same rated DC voltage value allows the transmitted power to match the third flexible DC converter valve 33. Due to voltage matching, the third flexible DC converter valve 33 does not require additional voltage regulation equipment to adapt to the input voltage and can quickly enter the working state, thereby stably starting the fourth flexible DC converter valve 34 and the entire system. This avoids start-up failure, start-up delay, or equipment impact caused by voltage mismatch, significantly improving the reliability of black starts, allowing the system to resume operation in the shortest possible time, and reducing the loss of new energy power caused by shutdowns.

[0054] exist Figure 1 In this process, the rated DC voltage of the second flexible DC converter valve 22 is the same as the rated DC voltage of the fourth flexible DC converter valve 34.

[0055] In this way, similar to how the rated DC voltage of the first flexible DC converter valve 21 is the same as that of the third flexible DC converter valve 33, the rated DC voltage of the second flexible DC converter valve 22 is the same as that of the fourth flexible DC converter valve 34. This means that when the receiving-end converter station 2 feeds back power to the fourth flexible DC converter valve 34 through the second flexible DC converter valve 22, the fourth flexible DC converter valve 34 can be started smoothly without an additional voltage regulation device. This avoids startup failures, delays, or equipment damage caused by voltage mismatch, improves black start reliability, shortens system recovery time, and reduces losses from new energy power outages.

[0056] like Figure 1 As shown, the sending-end converter station 3 also includes a first rectifier transformer 312, a second rectifier transformer 313, a third rectifier transformer 314, and a fourth rectifier transformer 315; the first diode valve 31 is connected to the sending-end AC bus 37 through the first rectifier transformer 312, the second diode valve 32 is connected to the sending-end AC bus 37 through the second rectifier transformer 313, the third diode valve 35 is connected to the sending-end AC bus 37 through the third rectifier transformer 314, and the fourth diode valve 36 is connected to the sending-end AC bus 37 through the fourth rectifier transformer 315.

[0057] In this way, the voltage level can be flexibly adjusted using a rectifier transformer to match the voltage between each diode valve and the sending-end AC bus 37. Furthermore, the electricity generated by new energy power generation may contain harmonics; the rectifier transformer can also filter and isolate these harmonics, reducing their impact on the diode valves and subsequent DC circuits, improving power quality, and allowing energy to be transferred more efficiently from the AC side to the DC side, thus enhancing the overall system's transmission performance. Simultaneously, the rectifier transformer also provides electrical isolation; in the event of a fault in the sending-end new energy source 4 or a diode valve, the rectifier transformer can prevent the fault from spreading, preventing the fault range from expanding, and improving the overall system's safety and reliability.

[0058] like Figure 1 As shown, the receiving-end converter station 2 also includes a first connecting transformer 29, a second connecting transformer 210, a third connecting transformer 211, and a fourth connecting transformer 212; the fifth flexible DC converter valve 23 is connected to the receiving-end AC bus 213 through the first connecting transformer 29, the first flexible DC converter valve 21 is connected to the receiving-end AC bus 213 through the second connecting transformer 210, the second flexible DC converter valve 22 is connected to the receiving-end AC bus 213 through the third connecting transformer 211, and the sixth flexible DC converter valve 24 is connected to the receiving-end AC bus 213 through the fourth connecting transformer 212.

[0059] In this embodiment, each flexible DC converter valve is connected to the receiving-end AC bus via its corresponding connecting transformer, which precisely matches the voltage levels of the converter valve and the receiving-end power grid 1. Different flexible DC converter valves have different output characteristics; the connecting transformer connected to the flexible DC converter valve can uniformly adapt its output voltage to the voltage of the receiving-end power grid 1, avoiding connection failures or energy losses due to voltage mismatch and ensuring a smooth transition of power from the converter valve to the grid. Furthermore, regarding fault isolation, if a flexible DC converter valve or connecting transformer fails, the faulty part can be isolated by disconnecting the corresponding connecting transformer, while other normal converter valves can still supply power to the grid normally, minimizing the scope of the fault's impact.

[0060] like Figure 1 As shown, the sending-end converter station 3 also includes a fifth connecting transformer 316 and a sixth connecting transformer 317; the third flexible DC converter valve 33 is connected to the sending-end AC bus 37 through the fifth connecting transformer 316, and the fourth flexible DC converter valve 34 is connected to the sending-end AC bus 37 through the sixth connecting transformer 317.

[0061] In this embodiment, the fifth and sixth connecting transformers can precisely match the voltage levels of the third and fourth flexible DC converter valves and the sending-end AC bus 37. Furthermore, if the third flexible DC converter valve 33 or the fifth connecting transformer 316 fails, the connection between the fifth connecting transformer 316 and the sending-end AC bus 37 or the third flexible DC converter valve 33 can be disconnected, isolating the faulty portion. At this time, the fourth flexible DC converter valve 34 can still operate normally through the sixth connecting transformer 317. Similarly, if the fourth flexible DC converter valve 34 or the sixth connecting transformer 317 fails, the third flexible DC converter valve 33 can continue to operate, reducing the scope of the fault's impact, decreasing the risk of system downtime, improving the reliability of the sending-end converter station 3, and ensuring the continuous transmission of new energy power.

[0062] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A new energy-based true bipolar hybrid DC transmission system, 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 via a bipolar band metal neutral line; the sending-end converter station is connected to the sending-end renewable energy source. The receiving-end converter station includes a first flexible DC converter valve, a second flexible DC converter valve, and a receiving-end AC bus; the first flexible DC converter valve and the second flexible DC converter valve are connected in series on the DC side, and the first flexible DC converter valve and the second flexible DC converter valve are respectively connected to the receiving-end AC bus on the AC side. The sending-end converter station includes a first diode valve, a second diode valve, a third flexible DC converter valve, a fourth flexible DC converter valve, a third diode valve, a fourth diode valve, and a sending-end AC bus; the first diode valve, the second diode valve, the third flexible DC converter valve, the fourth flexible DC converter valve, the third diode valve, and the fourth diode valve are respectively connected to the sending-end AC bus on the AC side; the sending-end AC bus is connected to the sending-end new energy source; The first diode valve, the second diode valve, the third flexible DC converter valve, the fourth flexible DC converter valve, the third diode valve, and the fourth diode valve are connected in series on the DC side; the first diode valve is connected to the receiving-end converter station through the positive pole transmission line, and a first bypass switch is connected between the third flexible DC converter valve and the positive pole transmission line; the fourth diode valve is connected to the receiving-end converter station through the negative pole transmission line; the third flexible DC converter valve and the fourth flexible DC converter valve are connected to the first flexible DC converter valve and the second flexible DC converter valve through a metal return line.

2. The system according to claim 1, characterized in that, The receiving-end converter station also includes a fifth flexible DC converter valve and a sixth flexible DC converter valve; the fifth flexible DC converter valve, the first flexible DC converter valve, the second flexible DC converter valve, and the sixth flexible DC converter valve are connected in series on the DC side, and connected in parallel on the AC side after passing through corresponding connecting transformers; The fifth flexible DC converter valve is connected to the positive power transmission line, and the sixth flexible DC converter valve is connected to the negative power transmission line; a first DC disconnect switch is connected between the first diode valve and the positive power transmission line, and a second DC disconnect switch is connected between the fifth flexible DC converter valve and the positive power transmission line; a second bypass switch is connected between the first flexible DC converter valve and the positive power transmission line.

3. The system according to claim 2, characterized in that, A third bypass switch is connected between the fourth flexible DC converter valve and the negative pole transmission line; a fourth bypass switch is connected between the second flexible DC converter valve and the negative pole transmission line.

4. The system according to claim 2 or 3, characterized in that, A third DC disconnect switch is connected between the fourth diode valve and the negative power transmission line, and a fourth DC disconnect switch is connected between the sixth flexible DC converter valve and the negative power transmission line.

5. The system according to claim 2, characterized in that, The rated DC voltage of the first flexible DC converter valve is the same as that of the third flexible DC converter valve.

6. The system according to claim 4, characterized in that, The rated DC voltage of the second flexible DC converter valve is the same as that of the fourth flexible DC converter valve.

7. The system according to claim 4, characterized in that, The sending-end converter station further includes a first rectifier transformer, a second rectifier transformer, a third rectifier transformer, and a fourth rectifier transformer; the first diode valve is connected to the sending-end AC bus through the first rectifier transformer, the second diode valve is connected to the sending-end AC bus through the second rectifier transformer, the third diode valve is connected to the sending-end AC bus through the third rectifier transformer, and the fourth diode valve is connected to the sending-end AC bus through the fourth rectifier transformer.

8. The system according to claim 7, characterized in that, The receiving-end converter station further includes a first connecting transformer, a second connecting transformer, a third connecting transformer, and a fourth connecting transformer; the fifth flexible DC converter valve is connected to the receiving-end AC bus via the first connecting transformer, the first flexible DC converter valve is connected to the receiving-end AC bus via the second connecting transformer, the second flexible DC converter valve is connected to the receiving-end AC bus via the third connecting transformer, and the sixth flexible DC converter valve is connected to the receiving-end AC bus via the fourth connecting transformer.

9. The system according to claim 8, characterized in that, The sending-end converter station also includes a fifth connecting transformer and a sixth connecting transformer; the third flexible DC converter valve is connected to the sending-end AC bus through the fifth connecting transformer, and the fourth flexible DC converter valve is connected to the sending-end AC bus through the sixth connecting transformer.

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