Direct-current power transmission system and fault ride-through method based on self-energy-storage and energy-consumption branch cooperation of MMC

Abstract

The invention discloses a DC power transmission system based on MMC self energy storage and energy consumption branch cooperation and a fault ride-through method. The DC power transmission system comprises a sending end converter station and a receiving end converter station. Each of the sending-end converter station and the receiving-end converter station comprises three bridge arms which are arranged between the positive-pole direct-current bus and the negative-pole direct-current bus and respectively correspond to the A phase, the B phase and the C phase; wherein each bridge arm comprises an upper sub-bridge arm and a lower sub-bridge arm; each sub-bridge arm of the receiving end converter station comprises N second units and a bridge arm inductor which are connected in series; the second unit comprises an MMC half-bridge sub-module and an energy consumption branch; the energy consumption branch comprises a thyristor and an energy consumption resistor which are connected in series; the positive electrode of the thyristor is connected to the upper port of the MMC half-bridge sub-module in the second unit, and the negative electrode of the thyristor is connected to the lower port of the MMC half-bridge sub-module in the second unit through the energy consumption resistor. The invention provides an economical and effective fault ride-through solution.

Description

Technical Field

[0001] This invention relates to the field of fault ride-through technology in DC power grids, specifically to a fault ride-through method and apparatus based on the coordination of MMC self-energy storage and energy-consuming branches. Background Technology

[0002] VSC-HVDC DC transmission systems based on modular multilevel converters (MMCs) have become the mainstream solution for long-distance power transmission. Figure 1 As shown, its advantages include low harmonic content, strong controllability, and high modularity. However, when a fault occurs on the DC side, overcurrent or overvoltage occurs. Traditional half-bridge circuits cannot reverse the current and must be blocked, leading to transmission interruption, DC voltage collapse, and even system shutdown, seriously threatening operational reliability. Against this backdrop, the fault ride-through capability of flexible DC transmission systems in the receiving-end grid has become one of the key bottlenecks for the further development of large-capacity offshore wind power systems.

[0003] Currently, typical fault ride-through measures mainly include the following categories: First, the DC energy dissipation resistor scheme, which involves connecting an energy dissipation resistor in parallel on the DC side of the converter station and absorbing fault energy through IGBT switching. However, this scheme requires large-capacity resistor devices, resulting in high heat dissipation costs. Second, the AC energy dissipation device scheme, which relies on AC circuit breaker switching resistors or STATCOM devices to dissipate energy. However, this scheme is affected by communication delays, with response times typically exceeding 10ms, resulting in insufficient dynamic performance. Existing fault ride-through strategies are prone to overprotection behavior when facing complex power grid faults, weakening the ability to support the normal state of the system and even leading to prolonged outages, significantly impacting the continuity and stability of power transmission. Third, the full-bridge or hybrid submodule scheme, which achieves self-clearing of fault current by improving the submodule topology. However, its power semiconductor device cost is more than 30% higher than traditional schemes, and the control system is also more complex.

[0004] During fault ride-through, timely energy consumption faces a timing matching challenge: the blocking of the half-bridge submodule causes a sharp drop in DC power, while the AC energy dissipation device must wait for the fault signal transmitted across stations before it can operate, resulting in the power surplus not being dissipated quickly and thus causing DC overvoltage. Furthermore, the existing solution lacks a dynamic coordination mechanism—there is a lack of coordination between the blocking of the half-bridge submodule and the commissioning of the energy dissipation device; after blocking, the system inertia decreases, exacerbating the risk of transient oscillations. From an economic perspective, the full-bridge / hybrid submodule solution significantly increases costs, while the DC energy dissipation resistor suffers from low equipment utilization due to long-term idleness, and the overall cost-effectiveness still needs improvement.

[0005] Therefore, there is an urgent need to provide improved fault-crossing methods and devices. Summary of the Invention

[0006] To address the shortcomings of existing technical solutions, this invention proposes a fault ride-through method and apparatus based on the coordination of MMC self-energy storage and energy dissipation branches. In the initial stage of fault ride-through, the number of MMC submodules put into operation is reduced according to the changes in the capacitor voltage of the MMC submodules to absorb the surplus power of the system and maintain the stability of the DC voltage. During fault ride-through, the number of MMC submodules put into operation remains unchanged, and the switching of the energy dissipation branches is controlled by thyristors to realize the absorption of surplus power, thus providing a low-cost solution to the fault ride-through problem.

[0007] According to a first aspect of the present invention, a flexible direct current (DC) transmission system is provided. The flexible DC transmission system includes: a sending-end converter station and a receiving-end converter station; The sending-end converter station and the receiving-end converter station each include three bridge arms located between the positive DC bus and the negative DC bus, respectively corresponding to A, B, and C; each bridge arm includes an upper sub-bridge arm and a lower sub-bridge arm. Each sub-bridge arm of the sending-end converter station includes series-connected... N A first unit and a bridge arm inductor; the first unit is an MMC half-bridge submodule; Each sub-bridge arm of the receiving-end converter station includes series-connected... N A second unit and a bridge arm inductor; the second unit includes an MMC half-bridge submodule and a power dissipation branch; the power dissipation branch includes a thyristor and a power dissipation resistor connected in series; the positive terminal of the thyristor is connected to the upper port of the MMC half-bridge submodule in the second unit, and the negative terminal of the thyristor is connected to the lower port of the MMC half-bridge submodule in the second unit via the power dissipation resistor.

[0008] Furthermore, the MMC half-bridge sub-module includes: a first power switch, a second power switch, a first diode, a second diode, and a capacitor; The upper port of the MMC half-bridge module is connected to the emitter of the first power switch and the collector of the second power switch, respectively, and the lower port of the MMC half-bridge module is connected to the emitter of the second power switch. The positive and negative terminals of the capacitor are connected to the collector of the first power switch and the emitter of the second power switch, respectively. The gates of the first power switch and the second power switch are respectively connected to the corresponding switching control signals; The positive and negative terminals of the first diode are connected to the emitter and collector of the first power switch, respectively. The positive and negative terminals of the second diode are connected to the emitter and collector of the second power switch, respectively.

[0009] Furthermore, the first power switch and the second switch are IGBTs, IGCTs, or IEGTs.

[0010] Furthermore, when the voltage between the upper and lower ports of the MMC half-bridge submodule in the second unit is greater than the first threshold, the thyristor is turned on, and the energy-consuming resistor is put into operation to absorb the surplus power of the system. When the voltage between the upper and lower ports of the MMC half-bridge submodule in the second unit is less than the second threshold, the thyristor is turned off and the energy-consuming resistor is cut off; the first threshold is greater than the second threshold.

[0011] Furthermore, the energy-consuming branch and the MMC half-bridge submodule in the second unit share a water-cooling system.

[0012] According to a second aspect of the present invention, a fault ride-through method based on MMC self-energy storage and energy dissipation branch coordination is provided. This method is used in the flexible DC transmission system as described in the first aspect of the present invention, and includes the following steps: Based on the DC voltage between the positive DC bus and the negative DC bus during normal operation of the flexible DC transmission system and the maximum safe value of the capacitor voltage in the second unit, the lower limit of the number of second units put into the flexible DC transmission system is determined. Real-time monitoring of the AC output voltage of the flexible DC transmission system and the capacitor voltage in each second unit; When a fault is detected at the AC output terminal, the target number of the second unit to be put into operation at the current time is determined based on the capacitor voltage in each second unit at the current time. If the target number of inputs is greater than the lower limit, then the number of second units input is equal to the target number of inputs; if the target number of inputs is less than or equal to the lower limit, then the number of second units input is equal to the lower limit. By utilizing the thyristors in the energy-consuming branch of the second unit, the energy-consuming branch is adaptively switched on or off based on the port voltage between the upper and lower ports of the MMC half-bridge submodule in the second unit, so as to absorb surplus power during fault ride-through.

[0013] Furthermore, based on the DC voltage between the positive and negative DC buses during normal operation of the flexible DC transmission system and the maximum safe value of the capacitor voltage in the second unit, the lower limit of the number of second units to be put into operation in the flexible DC transmission system is determined, including: The lower limit of the number of second units to be invested is calculated according to the following formula. :

[0014] in, The DC voltage is... This is the maximum safe value for the capacitor voltage in the second unit. Furthermore, the target number of units to be engaged at the current moment is determined based on the capacitor voltage in each unit at the current moment, including: Determine the average capacitor voltage at the current moment based on the capacitor voltage in each second unit at the current moment; Based on the DC voltage during normal operation of the flexible DC transmission system and the average capacitor voltage at the current moment, the target number of units to be put into operation at the current moment is obtained.

[0015] Furthermore, based on the DC voltage during normal operation of the flexible DC transmission system and the average capacitor voltage at the current moment, the target number of the second unit to be engaged at the current moment is obtained, including: The current time is determined according to the following formula. t The second unit's target input number :

[0016] in, The DC voltage is... For the current moment t The average capacitor voltage.

[0017] Furthermore, if the target number of inputs is greater than the lower limit, then the number of second units input is equal to the target number of inputs; if the target number of inputs is less than or equal to the lower limit, then the number of second units input is equal to the lower limit, including: When the target number of inputs or the lower limit of the number is less than the number of inputs at the previous moment, the second unit with the highest capacitor voltage is selected from the currently input second unit and cut off until the number of inputs of the second unit is equal to the target number of inputs or the lower limit of the number.

[0018] The beneficial effects of this invention are that, compared with the prior art, 1. In view of the shortcomings of existing DC power dissipation devices that use a large number of fully controlled devices, are independently arranged, and have high costs, this invention distributes the power dissipation branches to the ports of the onshore MMC submodules, adds power dissipation branches to the original half-bridge submodules, and uses thyristors as the core device to realize the power dissipation function. In conjunction with the switching of submodules, the surplus power can be controlled.

[0019] 2. By reducing the number of sub-modules connected to the system during the initial fault ride-through phase, damage to the system caused by excessive current can be effectively prevented, thereby maintaining the stability of DC voltage, effectively absorbing the system's surplus power, reducing current surges, and avoiding system overload.

[0020] 3. The system adopts a shared water cooling and power supply system with the energy-consuming branch and the MMC half-bridge module, and the thyristor replaces the expensive full control device, realizing the control of the energy-consuming unit, which greatly reduces the cost of fault ride-through of offshore wind power system and provides an economical and effective fault ride-through solution. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the architecture of a VSC-HVDC power transmission system based on a modular multilevel converter (MMC) in the existing technology. Figure 2 This is a schematic diagram of the topology of the VSC-HVDC DC transmission system based on the modular multilevel converter (MMC) of the present invention. Figure 3 The waveform diagrams of various physical quantities during fault ride-through are shown in the fault ride-through method based on MMC self-energy storage and energy dissipation branch coordination of the present invention. Detailed Implementation

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

[0023] Example 1 This embodiment provides a flexible DC transmission system constructed based on a modular multilevel converter (MMC). For example... Figure 2 The flexible DC transmission system in this embodiment includes a sending-end converter station and a receiving-end converter station.

[0024] The sending-end converter station and the receiving-end converter station each include three bridge arms, corresponding to A, B, and C respectively, which are arranged between the positive DC bus and the negative DC bus; each bridge arm includes an upper sub-bridge arm and a lower sub-bridge arm. Each sub-bridge arm of the sending-end converter station includes series-connected... N The first unit SM1, SM2, ..., SM N And bridge arm inductors; the first unit is an MMC half-bridge sub-module; Each sub-bridge arm of the receiving-end converter station includes series-connected... N The second unit EDU-SM1, EDU-SM2, ..., EDU-SM NThe second unit includes an MMC half-bridge submodule and a power dissipation branch; the power dissipation branch includes a thyristor and a power dissipation resistor connected in series; the positive terminal of the thyristor is connected to the upper port of the MMC half-bridge submodule in the second unit, and the negative terminal of the thyristor is connected to the lower port of the MMC half-bridge submodule in the second unit via the power dissipation resistor.

[0025] In this embodiment, to address the shortcomings of existing DC power dissipation devices that use a large number of fully controlled components, are independently arranged, and have high costs, the power dissipation branches are distributed to the ports of the onshore MMC submodules. Power dissipation branches are added to the original half-bridge submodules, and thyristors are used as the core components to enable the power dissipation function. In conjunction with the switching of submodules, the surplus power can be controlled.

[0026] Furthermore, each of the MMC half-bridge sub-modules in the first and second units includes: a first power switch, a second power switch, a first diode, a second diode, and a capacitor. The upper port of the MMC half-bridge sub-module is connected to the emitter of the first power switch and the collector of the second power switch, respectively, and the lower port of the MMC half-bridge sub-module is connected to the emitter of the second power switch. The positive and negative terminals of the capacitor are connected to the collector of the first power switch and the emitter of the second power switch, respectively. The gates of the first and second power switches are connected to corresponding switching control signals, respectively. The positive and negative terminals of the first diode are connected to the emitter and collector of the first power switch, respectively; the positive and negative terminals of the second diode are connected to the emitter and collector of the second power switch, respectively.

[0027] By controlling the state of the power switches in the MMC half-bridge submodule, the output capacitor voltage or zero level can be achieved. After multiple MMC half-bridge submodules are connected in series, a stepped AC voltage can be synthesized through precise switching and bypassing, realizing DC-AC power conversion, controlling the transmission power, and reducing output harmonics.

[0028] Preferably, the first power switch and the second switch are IGBTs, IGCTs, or IEGTs.

[0029] Furthermore, when the voltage between the upper and lower ports A and B of the MMC half-bridge submodule in the second unit is greater than the first threshold (e.g., 1.2 pu), the thyristor is turned on, and the energy-consuming resistor is connected to absorb the surplus power of the system; when the voltage between the upper and lower ports A and B of the MMC half-bridge submodule in the second unit is less than the second threshold (e.g., 0.9 pu), the thyristor is turned off, and the energy-consuming resistor is disconnected; the first threshold is greater than the second threshold.

[0030] By setting thyristor-controlled switching of energy-consuming resistors, expensive fully controlled devices can be replaced, significantly reducing the cost of fault ride-through in offshore wind power systems. Simultaneously, by setting a high input threshold and a low output threshold, frequent switching of energy-consuming branches due to minor fluctuations near voltage critical points can be effectively avoided, thus preventing oscillations, ensuring stable and reliable operation, and providing the necessary margin for the DC transmission system to maintain a continuous and stable power dissipation state during faults.

[0031] It should be noted that setting the first threshold to 1.2 pu and the second threshold to 0.9 pu is merely an example. In practice, the first and second thresholds can be adjusted according to the actual operating conditions, as long as they are within the safe voltage thresholds between the upper and lower ports A and B.

[0032] Preferably, the energy-consuming branch and the MMC half-bridge submodule in the second unit share a water-cooling system. This further reduces the cost of fault ride-through in offshore wind power systems, providing a more economical and effective fault ride-through solution.

[0033] Example 2 This embodiment provides a fault ride-through method based on MMC self-energy storage and energy dissipation branch coordination. This method is used in the flexible DC transmission system as described in the first aspect of this invention.

[0034] Specifically, the method includes the following steps: S1. Based on the DC voltage between the positive DC bus and the negative DC bus during normal operation of the flexible DC transmission system and the maximum safe value of the capacitor voltage in the second unit, determine the lower limit of the number of second units put into the flexible DC transmission system.

[0035] Assuming the system is operating normally, the number of second units in a single sub-arm of the receiving-end converter station is: N The initial capacitor voltage in the second unit is U The power that needs to be absorbed is P surplus The voltage change of the capacitor in the second unit is as follows: U Since the receiving-end converter station includes three bridge arms corresponding to A, B, and C, and each bridge arm includes an upper sub-bridge arm and a lower sub-bridge arm, it has a total of 6... N The second unit, therefore, during fault ride-through, the capacitor energy of the receiving-end converter station during this process. W C satisfy: (1) Thus, (2) and U It should be less than the device's safe operating voltage, that is, it should be less than the maximum safe value of the capacitor voltage in the second unit. U safe (e.g., 1.4 pu).

[0036] It is evident that, with the increase of fault ride-through time, assuming the number of second units remains constant, the changed capacitor voltage in the second unit... U 'It will gradually increase beyond the maximum safety value' U safe Therefore, it can be seen from equation (2) that in order to avoid exceeding the maximum safe value due to the infinite increase of capacitor voltage, U safe This can reduce the number of inputs required for the second unit.

[0037] At the same time, according to Figure 2 Given the topological relationships in the diagram, assuming the number of second units introduced is n, then the following conditions are met: (3) in, It is the DC voltage between the positive DC bus and the negative DC bus.

[0038] because U 'Cannot exceed the maximum safety value' U safe Therefore, the lower limit of the number of second units to be invested. n min It should be: (4) S2. Monitor the AC output voltage of the flexible DC transmission system and the capacitor voltage in each second unit in real time.

[0039] In this step, monitoring the AC output voltage aims to determine if an AC output fault has occurred. Monitoring the capacitor voltage in each second unit aims to better determine the number of second units that should be activated in subsequent steps.

[0040] S3. When a fault is detected at the AC output terminal, the target number of the second unit to be put into operation at the current time is determined based on the capacitor voltage in each second unit at the current time.

[0041] In this step, a fault at the AC output terminal can be detected by monitoring the AC output voltage ( Figure 2 In U out To make a judgment. For example, when U out Less than the first preset threshold (e.g., 0.8 pu) or Uout When the rate of decrease exceeds a second preset threshold (e.g., 0.2 pu / s), a fault is considered to have occurred at the AC output terminal.

[0042] Furthermore, in this step, the target number of units to be engaged at the current time is determined based on the capacitor voltage in each unit at the current time, specifically including: S31. Determine the average capacitor voltage at the current moment based on the capacitor voltage in each second unit at the current moment.

[0043] S32. Based on the DC voltage during normal operation of the flexible DC transmission system and the average capacitor voltage at the current moment, obtain the target number of the second unit to be put into operation at the current moment.

[0044] Specifically, the current time can be determined according to the following formula. t The second unit's target input number : (5) in, The DC voltage is... Let be the average capacitor voltage at the current time t.

[0045] S4. If the target number of inputs is greater than the lower limit of the number, then the number of second units input is equal to the target number of inputs; if the target number of inputs is less than or equal to the lower limit of the number, then the number of second units input is equal to the lower limit of the number.

[0046] This step allows for the orderly reduction of the number of sub-modules, which in turn increases the capacitor voltage in the second unit sequentially, ultimately absorbing the surplus power. On the other hand, it ensures that the capacitor voltage does not exceed the maximum safe value (e.g., 1.4 pu).

[0047] S5. By utilizing the thyristors in the energy-consuming branch of the second unit, the energy-consuming branch is adaptively put into or taken out according to the port voltage between the upper and lower ports of the MMC half-bridge circuit in the second unit, so as to realize the absorption of surplus power during fault ride-through.

[0048] Specifically, when the voltage between the upper and lower ports A and B of the MMC half-bridge submodule in the second unit is greater than a first threshold (e.g., 1.2 pu), the thyristor in the power dissipation branch is turned on, and the power dissipation resistor is engaged to absorb the system's surplus power. When the voltage between the upper and lower ports A and B of the MMC half-bridge submodule in the second unit is less than a second threshold (e.g., 0.9 pu), the thyristor in the power dissipation branch is turned off, and the power dissipation resistor is disconnected. In this way, the power dissipation branch can be adaptively engaged or disengaged based on the port voltage between the upper and lower ports to absorb surplus power during fault ride-through.

[0049] like Figure 3 To further illustrate the present invention, its working process is now described in detail below: t Before time 0, the system is in normal operation, the power-consuming branch is not working, and the MMC works according to normal control logic.

[0050] t At time 0, a fault occurs, and the AC side voltage rapidly drops to 0. The system's power output capacity decreases, and the surplus power charges the capacitors in the system, causing the system's DC voltage to rise. At this point, the number of submodules deployed in the MMC is determined based on the change in capacitor voltage in the system after the fault, thereby systematically reducing the number of submodules deployed.

[0051] During this period, referring to equation (5), the value of the reduced second unit can be calculated based on the capacitor voltage, and the number can be reduced in an orderly manner to the lower limit. n min Then, keeping the number of inputs in the second unit unchanged, the surplus power in the system is finally absorbed.

[0052] t At time 1, DC voltage The system's set threshold has been reached.

[0053] t At time 2, the number of second units deployed in the system remains unchanged, still at the lower limit. n min .

[0054] Assuming each submodule of the bridge arm is configured with a power-consuming branch, that is, each bridge arm contains n A second unit is configured with a power dissipation branch. When the port voltage of the MMC half-bridge submodule in the second unit is greater than 1.2 pu, the thyristor is turned on, the power dissipation resistor is connected, and the excess power of the system is absorbed; when the port voltage of the MMC half-bridge submodule in the second unit is less than 0.9 pu, the thyristor is turned off, and the power dissipation resistor is disconnected.

[0055] t At time 3, the energy-consuming branch completely shuts down.

[0056] Then, repeat the above process, and the system DC voltage will fluctuate within the rated range.

[0057] t At time 4, the AC side voltage begins to recover, and the surplus power gradually decreases. A power overshoot occurs at the moment the onshore converter station recovers its grid-connected power, causing the system DC voltage to drop below its rated value. At this time, the energy-consuming branch begins to disconnect. It begins to descend, according to equation (5), as As the number of units decreased, the number of units in the second unit began to gradually increase.

[0058] t At time 5, the energy-consuming branch is completely disconnected, and the system DC voltage... The system gradually recovered to near its rated value, after which it operated normally, and the system submodules returned to normal operation.

[0059] In summary, the beneficial effects of the present invention are that, compared with the prior art, 1. In view of the shortcomings of existing DC power dissipation devices that use a large number of fully controlled devices, are independently arranged, and have high costs, this invention distributes the power dissipation branches to the ports of the onshore MMC submodules, adds power dissipation branches to the original half-bridge submodules, and uses thyristors as the core device to realize the power dissipation function. In conjunction with the switching of submodules, the surplus power can be controlled.

[0060] 2. By reducing the number of sub-modules connected to the system during the initial fault ride-through phase, damage to the system caused by excessive current can be effectively prevented, thereby maintaining the stability of DC voltage, effectively absorbing the system's surplus power, reducing current surges, and avoiding system overload.

[0061] 3. The system adopts a shared water cooling and power supply system with the energy-consuming branch and the MMC half-bridge module, and the thyristor replaces the expensive full control device, realizing the control of the energy-consuming unit, which greatly reduces the cost of fault ride-through of offshore wind power system and provides an economical and effective fault ride-through solution.

[0062] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A DC transmission system based on MMC self-energy storage and energy dissipation branch coordination, characterized in that, include: Sending-end converter station and receiving-end converter station; The sending-end converter station and the receiving-end converter station each include three bridge arms located between the positive DC bus and the negative DC bus, respectively corresponding to A, B, and C; each bridge arm includes an upper sub-bridge arm and a lower sub-bridge arm. Each sub-bridge arm of the sending-end converter station includes series-connected... N A first unit and a bridge arm inductor; the first unit is an MMC half-bridge submodule; Each sub-bridge arm of the receiving-end converter station includes series-connected... N A second unit and a bridge arm inductor; the second unit includes an MMC half-bridge submodule and a power dissipation branch; the power dissipation branch includes a thyristor and a power dissipation resistor connected in series; the positive terminal of the thyristor is connected to the upper port of the MMC half-bridge submodule in the second unit, and the negative terminal of the thyristor is connected to the lower port of the MMC half-bridge submodule in the second unit via the power dissipation resistor.

2. The DC transmission system based on MMC self-energy storage and energy dissipation branch coordination according to claim 1, characterized in that, The MMC half-bridge submodule includes: a first power switch, a second power switch, a first diode, a second diode, and a capacitor; The upper port of the MMC half-bridge module is connected to the emitter of the first power switch and the collector of the second power switch, respectively, and the lower port of the MMC half-bridge module is connected to the emitter of the second power switch. The positive and negative terminals of the capacitor are connected to the collector of the first power switch and the emitter of the second power switch, respectively. The gates of the first power switch and the second power switch are respectively connected to the corresponding switching control signals; The positive and negative terminals of the first diode are connected to the emitter and collector of the first power switch, respectively. The positive and negative terminals of the second diode are connected to the emitter and collector of the second power switch, respectively.

3. The DC transmission system based on MMC self-energy storage and energy dissipation branch coordination according to claim 2, characterized in that, The first power switch and the second switch are IGBTs, IGCTs, or IEGTs.

4. The DC transmission system based on MMC self-energy storage and energy dissipation branch coordination according to claim 1, characterized in that, When the voltage between the upper and lower ports of the MMC half-bridge submodule in the second unit is greater than the first threshold, the thyristor is turned on and the energy-consuming resistor is put into operation to absorb the surplus power of the system. When the voltage between the upper and lower ports of the MMC half-bridge submodule in the second unit is less than the second threshold, the thyristor is turned off and the energy-consuming resistor is disconnected. The first threshold is greater than the second threshold.

5. The DC transmission system based on MMC self-energy storage and energy dissipation branch coordination according to claim 1, characterized in that, The energy-consuming branch and the MMC half-bridge module in the second unit share a water-cooling system.

6. A fault ride-through method based on MMC self-energy storage and energy dissipation branch coordination, used in a DC transmission system based on MMC self-energy storage and energy dissipation branch coordination as described in any one of claims 1 to 5, characterized in that, The method includes: Based on the DC voltage between the positive DC bus and the negative DC bus during normal operation of the flexible DC transmission system and the maximum safe value of the capacitor voltage in the second unit, the lower limit of the number of second units put into the flexible DC transmission system is determined. Real-time monitoring of the AC output voltage of the flexible DC transmission system and the capacitor voltage in each second unit; When a fault is detected at the AC output terminal, the target number of the second unit to be put into operation at the current time is determined based on the capacitor voltage in each second unit at the current time. If the target number of inputs is greater than the lower limit, then the number of second units input is equal to the target number of inputs; if the target number of inputs is less than or equal to the lower limit, then the number of second units input is equal to the lower limit. By utilizing the thyristors in the energy-consuming branch of the second unit, the energy-consuming branch is adaptively put into or taken out based on the port voltage between the upper and lower ports of the MMC half-bridge circuit in the second unit, so as to realize the absorption of surplus power during fault ride-through.

7. The fault ride-through method based on MMC self-energy storage and energy dissipation branch coordination as described in claim 6, characterized in that, Based on the DC voltage between the positive and negative DC buses during normal operation of the flexible DC transmission system and the maximum safe value of the capacitor voltage in the second unit, the lower limit of the number of second units to be put into operation in the flexible DC transmission system is determined, including: The lower limit of the number of second units to be invested is calculated according to the following formula. : in, The DC voltage is... This is the maximum safe value for the capacitor voltage in the second unit.

8. The fault ride-through method based on MMC self-energy storage and energy dissipation branch coordination as described in claim 6, characterized in that, The target number of units to be deployed at the current time is determined based on the capacitor voltage in each unit at the current time, including: Determine the average capacitor voltage at the current moment based on the capacitor voltage in each second unit at the current moment; Based on the DC voltage during normal operation of the flexible DC transmission system and the average capacitor voltage at the current moment, the target number of units to be put into operation at the current moment is obtained.

9. The fault ride-through method based on MMC self-energy storage and energy dissipation branch coordination as described in claim 8, characterized in that, Based on the DC voltage during normal operation of the flexible DC transmission system and the average capacitor voltage at the current moment, the target number of units to be put into operation at the current moment is obtained, including: The current time is determined according to the following formula. t The second unit's target input number : in, The DC voltage is... For the current moment t The average capacitor voltage.

10. The fault ride-through method based on MMC self-energy storage and energy dissipation branch coordination as described in claim 8, characterized in that, If the target number of inputs is greater than the lower limit, then the number of second units input is equal to the target number of inputs; if the target number of inputs is less than or equal to the lower limit, then the number of second units input is equal to the lower limit, including: When the target number of inputs or the lower limit of the number is less than the number of inputs at the previous moment, the second unit with the highest capacitor voltage is selected from the currently input second unit and cut off until the number of inputs of the second unit is equal to the target number of inputs or the lower limit of the number.

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