A direct current system fault ride-through method and system

Abstract

The present application belongs to the field of direct current transmission and power electronics, and particularly relates to a kind of direct current system fault ride-through method and system.The present application is additionally provided with parallel bypass branch at controllable turn-off thyristor converter valve, when the AC system of receiving end of direct current system fails, according to different fault degree, different fault ride-through strategy is adopted, in mild fault, only through turn-off converter valve to assist commutation, in serious fault, bypass branch is turned on, controllable turn-off thyristor converter valve in controllable turn-off converter module is short-circuited, controllable turn-off thyristor converter valve is turned off, indirectly assists commutation, so that equipment can be adaptively protected from the influence of large current turn-off peak overvoltage, when serious fault occurs in receiving end power grid, fault ride-through can be completed, and converter valve can be ensured to operate stably during fault;And controllable turn-off thyristor and ordinary diode are mixed in series on bypass branch, to ensure large current carrying capacity, while reducing fault ride-through cost.

Description

Technical Field

[0001] This invention belongs to the field of DC power transmission and power electronics technology, specifically relating to a DC system fault ride-through method and system. Background Technology

[0002] The increased capacity and number of ultra-high-voltage direct current (UHVDC) lines have led to a pronounced "strong DC, weak AC" characteristic in the power grid. This is mainly reflected in two aspects: First, the number of bundled wind and thermal power transmission projects at the sending end of the grid is increasing, resulting in higher coupling between large-scale wind power and DC. The construction of supporting thermal power units is lagging behind, and AC faults at the receiving end can cause severe overvoltages at the sending end, further leading to wind turbine disconnection. Second, the receiving end grid is mostly located in load centers, with concentrated DC feed points. A single DC commutation failure can affect the stability of the receiving end grid and may lead to multiple DC commutation failures, further impacting the receiving end grid and causing potential hazards such as overspeeding of some thermal power units and exceeding power limits in some sections. In summary, faults in the receiving end grid have a significant impact on the sending and receiving end AC systems, and can simultaneously cause grid instability at both ends.

[0003] The rapid development of modern power systems and the upgrading of power electronics technology have led to significant progress in the technology of controllable turn-off current source converters. Improved voltage and overcurrent withstand capabilities of these devices have made their application in ultra-high voltage direct current (UHVDC) systems possible. DC systems based on controllable turn-off current source converters can solve the problem of inverter-side commutation failure: by placing the controllable turn-off converter on the inverter side, such as... Figure 1 As shown in the schematic diagram of the DC transmission system topology, the left half is the sending-end grid corresponding to the rectifier side, and the right half is the receiving-end grid corresponding to the inverter side. When the receiving-end grid corresponding to the inverter side is disturbed, the insufficient arc extinction angle margin will cause the commutation current to fail to be reduced to zero by relying on the reverse voltage of natural commutation. At this time, the commutation current is forcibly blocked by the turn-off capability of the controllable turn-off converter (controllable turn-off converter valve) to complete the commutation process, avoid the occurrence of commutation failure, thereby avoiding the impact of short-term faults in the AC system on DC transmission and realizing fault ride-through.

[0004] However, when a severe asymmetrical fault occurs in the receiving-end power grid, the current method of directly shutting off the controllable shut-off converter valve has several drawbacks. First, the surge in fault current makes it impossible for the controllable shut-off converter valve to forcibly shut off the current, resulting in commutation failure. Second, the forced shutdown of a large current will also lead to severe valve-side overvoltage problems, placing higher demands on surge arresters and converter valve equipment. Furthermore, when a severe three-phase low-voltage fault occurs in the receiving-end power grid, active power transmission cannot be maintained. The forced commutation of the controllable shut-off converter valve is not only ineffective for the AC / DC system and cannot complete fault ride-through, but also increases shutdown losses. Summary of the Invention

[0005] The purpose of this invention is to provide a DC system fault ride-through method and system to solve the problem that the existing method of directly shutting off the controllable shut-off converter valve cannot complete fault ride-through when a severe asymmetrical fault occurs in the receiving-end power grid.

[0006] To achieve the above objectives, the present invention provides a fault ride-through method for a DC system. The receiving end of the DC system is equipped with a controllable shutdown converter module. The controllable shutdown converter module includes a controllable shutdown thyristor converter valve and a bypass branch connected in parallel with the converter valve. Multiple controllable shutdown thyristors are connected in series on the bypass branch, and all controllable shutdown thyristors on the bypass branch are in the same direction as the controllable shutdown thyristors in the controllable shutdown thyristor converter valve.

[0007] The DC system fault ride-through method includes: real-time acquisition of the receiving-end grid side voltage and the receiving-end grid side zero-sequence voltage, and judgment on whether a fault has occurred in the AC system at the receiving end of the DC system and the degree of the fault.

[0008] When a fault occurs in the AC system at the receiving end of the DC system, different fault ride-through strategies are adopted according to the different fault degrees.

[0009] The beneficial effects of the proposed technical solution are as follows: A bypass branch including a controllable turn-off thyristor is connected in parallel to the controllable turn-off thyristor converter valve. When the controllable turn-off thyristor in the bypass branch is turned on, the controllable turn-off thyristor converter valve in the controllable turn-off converter module is short-circuited. The fault current is diverted through the bypass branch, making the current at the controllable turn-off thyristor converter valve in the controllable turn-off converter module zero, thus naturally turning off and completing the commutation process. This solves the problem that the method of directly turning off the controllable turn-off converter valve cannot complete fault ride-through in the event of a severe asymmetrical fault in the receiving-end power grid.

[0010] Furthermore, the specific method for adopting different fault-crossing strategies based on different fault degrees is as follows:

[0011] When the voltage amplitude on the receiving end network side is greater than the first set threshold and less than the second set threshold, and the zero-sequence voltage on the receiving end network side is greater than the third set threshold and less than the fourth set threshold, it is determined that an AC low voltage fault has occurred in the AC system of the receiving end. The controllable turn-off thyristor on the bypass branch is kept off, and the turn-off pulse of the controllable turn-off thyristor converter valve is enabled to turn off the controllable turn-off thyristor converter valve in the controllable turn-off converter module.

[0012] When the voltage amplitude on the receiving end network side is less than the first set threshold, or the zero-sequence voltage on the receiving end network side is greater than the fourth set threshold, it is determined that a serious three-phase fault or asymmetrical fault has occurred in the AC system at the receiving end. The controllable turn-off thyristor on the bypass branch is turned on to short-circuit the controllable turn-off thyristor converter valve in the controllable turn-off converter module, and the fault current is diverted through the bypass branch.

[0013] The first set threshold is less than the second set threshold; the third set threshold is less than the fourth set threshold.

[0014] The beneficial effects of the proposed technical solution are as follows: when a fault occurs in the AC system at the receiving end of the DC system, different fault ride-through strategies are adopted according to different fault degrees. That is, in the case of a minor fault, commutation is assisted only by turning off the controllable thyristor converter valve. In the case of a severe fault, the controllable thyristor converter valve is turned off through a bypass branch to indirectly assist commutation. Therefore, on the one hand, it can adaptively ensure that the equipment is not affected by the peak overvoltage of the large current shutdown, and on the other hand, it can ensure that the converter valve operates stably and reliably during the fault period.

[0015] Furthermore, a diode is also connected in series on the bypass branch; all the diodes are in the same direction as the controllable turn-off thyristor on the bypass branch.

[0016] The beneficial effect of the proposed technical solution is that it can reduce the cost of implementing the DC system fault ride-through method while ensuring high current carrying capacity.

[0017] Furthermore, the DC system fault ride-through method further includes: after turning on the controllable turn-off thyristor on the bypass branch, when the voltage amplitude of the receiving end grid side is greater than the second set threshold, the zero-sequence voltage of the receiving end grid is less than the third set threshold, and the deviation between the current DC current of the DC system and the DC current of the DC system under normal conditions is greater than the set deviation threshold, the controllable turn-off thyristor on the bypass branch is turned off, and the turn-off pulse of the controllable turn-off thyristor converter valve is enabled.

[0018] When the voltage amplitude of the receiving-end grid is greater than the second set threshold, the zero-sequence voltage of the receiving-end grid is less than the third set threshold, and the deviation between the current DC current of the DC system and the DC current of the DC system under normal conditions is less than the set deviation threshold, the turn-off pulse of the controllable turn-off thyristor converter valve is not enabled after a set delay time.

[0019] The beneficial effects of the proposed technical solution are as follows: when the AC fault is about to be eliminated but has not been completely eliminated, the bypass branch is first deactivated but the turn-off pulse enable of the controllable turn-off thyristor converter valve is still controlled, which can ensure that no commutation failure occurs during the power grid restoration process; after the AC fault is completely eliminated, the turn-off pulse enable signal of the controllable turn-off thyristor converter valve is deactivated in time, thereby reducing the turn-off loss of the converter valve.

[0020] Furthermore, when a severe three-phase fault or asymmetrical fault is detected in the AC system at the receiving end and the controllable turn-off thyristor on the bypass branch is turned on, the rectifier side is controlled to maintain a constant DC current operation.

[0021] The beneficial effects of the proposed technical solution are: maintaining current transmission during fault ride-through, ensuring that no transient overvoltage occurs on the rectifier side, and maintaining the stability of the AC system at the sending end.

[0022] The present invention also provides a DC system fault ride-through system, including a controllable shutdown converter module disposed at the receiving end of the DC system. The controllable shutdown converter module includes a controllable shutdown thyristor converter valve and a bypass branch connected in parallel with the converter valve. A plurality of controllable shutdown thyristors are connected in series on the bypass branch, and the controllable shutdown thyristors on the bypass branch are all in the same direction as the controllable shutdown thyristors in the controllable shutdown thyristor converter valve.

[0023] The DC system fault ride-through system also includes a processor for executing program instructions to implement the DC system fault ride-through method; the DC system fault ride-through method includes: real-time acquisition of the receiving-end grid-side voltage and the receiving-end grid-side zero-sequence voltage, and judgment on whether a fault has occurred in the AC system at the receiving end of the DC system and the degree of the fault.

[0024] When a fault occurs in the AC system at the receiving end of the DC system, different fault ride-through strategies are adopted according to different fault severity.

[0025] The beneficial effects of the proposed technical solution are as follows: A bypass branch including a controllable turn-off thyristor is connected in parallel to the controllable turn-off thyristor converter valve. When the controllable turn-off thyristor in the bypass branch is turned on, the controllable turn-off thyristor converter valve in the controllable turn-off converter module is short-circuited. The fault current is diverted through the bypass branch, making the current at the controllable turn-off thyristor converter valve in the controllable turn-off converter module zero, thus naturally turning off and completing the commutation process. This solves the problem that the method of directly turning off the controllable turn-off converter valve cannot complete fault ride-through in the event of a severe asymmetrical fault in the receiving-end power grid.

[0026] Furthermore, the specific method for adopting different fault-crossing strategies based on different fault degrees is as follows:

[0027] When the voltage amplitude on the receiving end network side is greater than the first set threshold and less than the second set threshold, and the zero-sequence voltage on the receiving end network side is greater than the third set threshold and less than the fourth set threshold, it is determined that an AC low voltage fault has occurred in the AC system of the receiving end. The controllable turn-off thyristor on the bypass branch is kept off, and the turn-off pulse of the controllable turn-off thyristor converter valve is enabled to turn off the controllable turn-off thyristor converter valve in the controllable turn-off converter module.

[0028] When the voltage amplitude on the receiving end grid side is less than the first set threshold, or the zero-sequence voltage on the receiving end grid side is greater than the fourth set threshold, it is determined that there is a serious three-phase fault or asymmetrical fault in the AC system at the receiving end. The controllable turn-off thyristor on the bypass branch is turned on to short-circuit the controllable turn-off thyristor converter valve in the controllable turn-off converter module, and the fault current is diverted through the bypass branch.

[0029] The first set threshold is less than the second set threshold; the third set threshold is less than the fourth set threshold.

[0030] The beneficial effects of the proposed technical solution are as follows: when a fault occurs in the AC system at the receiving end of the DC system, different fault ride-through strategies are adopted according to different fault degrees. That is, in the case of a minor fault, commutation is assisted only by turning off the controllable thyristor converter valve. In the case of a severe fault, the controllable thyristor converter valve is turned off through a bypass branch to indirectly assist commutation. Therefore, on the one hand, it can adaptively ensure that the equipment is not affected by the peak overvoltage of the large current shutdown, and on the other hand, it can ensure that the converter valve operates stably and reliably during the fault period.

[0031] Furthermore, a diode is also connected in series on the bypass branch; all the diodes are in the same direction as the controllable turn-off thyristor on the bypass branch.

[0032] The beneficial effect of the proposed technical solution is that it can reduce the cost of implementing the DC system fault ride-through method while ensuring high current carrying capacity.

[0033] Furthermore, the DC system fault ride-through method further includes: after turning on the controllable turn-off thyristor on the bypass branch, when the voltage amplitude of the receiving end grid side is greater than the second set threshold, the zero-sequence voltage of the receiving end grid is less than the third set threshold, and the deviation between the current DC current of the DC system and the DC current of the DC system under normal conditions is greater than the set deviation threshold, the controllable turn-off thyristor on the bypass branch is turned off, and the turn-off pulse of the controllable turn-off thyristor converter valve is enabled.

[0034] When the voltage amplitude of the receiving-end grid is greater than the second set threshold, the zero-sequence voltage of the receiving-end grid is less than the third set threshold, and the deviation between the current DC current of the DC system and the DC current of the DC system under normal conditions is less than the set deviation threshold, the turn-off pulse of the controllable turn-off thyristor converter valve is not enabled after a set delay time.

[0035] The beneficial effects of the proposed technical solution are as follows: when the AC fault is about to be eliminated but has not been completely eliminated, the bypass branch is first deactivated but the turn-off pulse enable of the controllable turn-off thyristor converter valve is still controlled, which can ensure that no commutation failure occurs during the power grid restoration process; after the AC fault is completely eliminated, the turn-off pulse enable signal of the controllable turn-off thyristor converter valve is deactivated in time, thereby reducing the turn-off loss of the converter valve.

[0036] Furthermore, the processor is also used to control the rectifier side to maintain a constant DC current operation when it determines that a serious three-phase fault or asymmetrical fault has occurred in the AC system at the receiving end and turns on the controllable turn-off thyristor on the bypass branch.

[0037] The beneficial effects of the proposed technical solution are: maintaining current transmission during fault ride-through, ensuring that no transient overvoltage occurs on the rectifier side, and maintaining the stability of the AC system at the sending end. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the topology of a DC transmission system including a controllable shutdown converter in the background art of this invention;

[0039] Figure 2 This is a schematic diagram of the three-phase topology of the controllable shutdown converter module in an embodiment of the DC system fault ride-through method of the present invention;

[0040] Figure 3 This is a logic block diagram of the fault ride-through method in an embodiment of the DC system fault ride-through method of the present invention;

[0041] Figure 4 This is a schematic diagram of the control function of the DC system to which the fault ride-through method of the present invention is applicable. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0043] Example of DC system fault ride-through method:

[0044] This embodiment provides a technical solution for a DC system fault ride-through method. In this embodiment, the receiving end of the DC system is equipped with a controllable shutdown converter module, such as... Figure 2 As shown, the controllable turn-off converter module includes a controllable turn-off thyristor converter valve and a bypass branch connected in parallel with the converter valve; the bypass branch is... Figure 2 The bypass branch has multiple controllable turn-off thyristors connected in series, and all the controllable turn-off thyristors in this bypass branch are in the same direction as the controllable turn-off thyristors in the controllable turn-off thyristor converter valve. Diodes are also connected in series on the bypass branch, and these diodes are all in the same direction as the controllable turn-off thyristors in the bypass branch. This bypass branch is mainly used to allow fault current to pass through. The more thyristors connected in series on the bypass branch, the stronger its ability to carry large currents. However, a large number of controllable turn-off thyristors connected in series would significantly increase the cost of the controllable turn-off converter module. Therefore, to reduce costs, controllable turn-off thyristors and ordinary diodes are mixed and connected in series, which can reduce the cost of implementing the DC system fault ride-through method while ensuring high current carrying capacity.

[0045] Specific DC system fault ride-through methods are as follows: Figure 3 As shown, it includes: real-time acquisition of the receiving-end grid-side voltage and the receiving-end grid-side zero-sequence voltage, and judgment on whether a fault has occurred in the AC system at the receiving end of the DC system and the degree of the fault.

[0046] As attached Figure 4 As shown, the DC transmission control strategy including the controllable turn-off converter module still adopts the control strategy of constant current on the rectifier side and constant voltage on the inverter side. Under steady-state conditions, the commutation bridge arm of the controllable turn-off converter valve is subjected to reverse commutation voltage, and the current naturally turns off after reaching 0.

[0047] When a fault is detected in the AC system at the receiving end of the DC system, different fault ride-through strategies are adopted according to the different fault degrees, as follows:

[0048] ① When the voltage amplitude on the receiving end network side is greater than the first set threshold and less than the second set threshold, and the zero-sequence voltage on the receiving end network side is greater than the third set threshold and less than the fourth set threshold, it is determined that the AC system at the receiving end has an AC low voltage fault. At this time, only the turn-off pulse of the controllable turn-off thyristor converter valve is enabled, and the bypass branch is not engaged. That is, the controllable turn-off thyristor on the bypass branch is kept off, which is equivalent to directly turning off the controllable turn-off thyristor converter valve in the controllable turn-off converter module through the turn-off pulse, forcibly blocking the commutation current to complete the commutation process.

[0049] ② When the voltage amplitude on the receiving end grid side is less than the first set threshold, or the zero-sequence voltage on the receiving end grid side is greater than the fourth set threshold, it is determined that a severe three-phase fault or asymmetrical fault (such as a metallic single-phase / multi-phase ground fault) has occurred in the AC system at the receiving end. Such severe faults will generate a large fault current, causing the controllable turn-off thyristor converter valve to be difficult to turn off due to the impact of a large current, and may even cause damage to the converter valve. Therefore, at this time, the bypass branch is activated, that is, the controllable turn-off thyristor on the bypass branch is turned on to short-circuit the controllable turn-off thyristor converter valve in the controllable turn-off converter module. The fault current is diverted through the bypass branch, so that the current at the controllable turn-off thyristor converter valve in the controllable turn-off converter module is 0, and the valve turns off naturally, completing the commutation process. While activating the bypass valve group, the rectifier side is kept running at a constant DC current, and the current reference value is kept constant at the steady-state current reference value. During the fault ride-through, the current transmission is maintained to ensure that no transient overvoltage occurs on the rectifier side and to maintain the stability of the sending end AC system.

[0050] After the controllable turn-off thyristor on the bypass branch is turned on, when the voltage amplitude on the receiving end grid side is greater than the second set threshold, the zero-sequence voltage of the receiving end grid is less than the third set threshold, and the deviation between the current DC current of the DC system and the DC current of the DC system under normal conditions is greater than the set deviation threshold, it indicates that the AC fault is about to be eliminated and the receiving end grid is about to return to steady-state operation. However, the fault has not been completely eliminated. Therefore, the bypass branch is first turned off, that is, the controllable turn-off thyristor on the bypass branch is turned off. However, the turn-off pulse of the controllable turn-off thyristor converter valve is still controlled to ensure that no commutation failure occurs during the grid restoration process.

[0051] When the voltage amplitude of the receiving-end grid is greater than the second set threshold, the zero-sequence voltage of the receiving-end grid is less than the third set threshold, and the deviation between the current DC current of the DC system and the DC current of the DC system under normal conditions is less than the set deviation threshold, it indicates that the AC fault has been eliminated and the receiving-end grid has fully recovered to steady-state operation. At this time, there is no need to assist in successful commutation through forced shutdown. Therefore, after a set delay time, the shutdown pulse of the controllable shutdown thyristor converter valve can be deactivated, that is, the controllable shutdown thyristor converter valve shutdown pulse enable signal is exited, thereby reducing the shutdown loss of the converter valve. In this embodiment, the set delay time is 1 second. In other embodiments, this delay time can be adaptively set according to the actual situation. During this set delay time, it is continuously judged whether the current system condition meets the conditions of "the voltage amplitude of the receiving-end grid is greater than the second set threshold, the zero-sequence voltage of the receiving-end grid is less than the third set threshold, and the deviation between the current DC current of the DC system and the DC current of the DC system under normal conditions is less than the set deviation threshold". Therefore, it can ensure that the AC system fault is eliminated and the DC system is operating in steady state, and ensure that the AC / DC system will not be disturbed after the converter valve shutdown signal is exited.

[0052] In the above-mentioned DC system fault ride-through method, the first set threshold is less than the second set threshold, and the third set threshold is less than the fourth set threshold. In this embodiment, the first set threshold is set to 0.4 pu, and the second set threshold is set to 0.9 pu. To ensure the sensitivity of fault detection and fault severity classification, the third set threshold is set to 0.1 pu, the fourth set threshold is set to 0.3 pu, and the set deviation threshold is set to 0.01 pu. Here, the unit pu represents the ratio of the actual value of a parameter at present to the value of the parameter under normal conditions.

[0053] Since the different fault ride-through strategies implemented through the controllable shutdown commutator module in this embodiment can adaptively assist commutation according to the severity of the fault, they can compensate for the defect that the commutation current cannot be reduced to zero by the reverse voltage of natural commutation due to insufficient margin in the arc-extinguishing angle setting. Therefore, the arc-extinguishing angle setting does not need to be too large. Figure 4 The arc extinction angle corresponding to ALPmax in the PI controller of the inverter station can be advanced by about 5° to 6° compared with the existing technology, which can effectively increase the power factor of active power transmission and reduce power loss.

[0054] Example of a DC system fault ride-through system:

[0055] This embodiment also provides a DC system fault ride-through system, which includes a controllable shutdown converter module disposed at the receiving end of the DC system. The controllable shutdown converter module includes a controllable shutdown thyristor converter valve and a bypass branch connected in parallel with the converter valve; the bypass branch is... Figure 2The BP branch in the circuit has multiple controllable turn-off thyristors connected in series. The controllable turn-off thyristors on the bypass branch are all in the same direction as the controllable turn-off thyristors in the controllable turn-off thyristor converter valve.

[0056] The DC system fault ride-through system also includes a processor for executing program instructions to implement the DC system fault ride-through method; the implemented DC system fault ride-through method includes: real-time acquisition of the receiving-end grid side voltage and the receiving-end grid side zero-sequence voltage, and judgment on whether the AC system at the receiving end of the DC system has a fault and the degree of the fault.

[0057] When a fault occurs in the AC system at the receiving end of the DC system, different fault ride-through strategies are adopted according to the different fault degrees.

[0058] Since the specific DC system fault ride-through method implemented by the fault ride-through system in this embodiment has been described in detail in the above-described DC system fault ride-through method embodiments, it will not be repeated here.

[0059] This invention has the following characteristics:

[0060] 1) A bypass branch including a controllable turn-off thyristor is connected in parallel to the controllable turn-off thyristor converter valve. In the event of a severe fault, the controllable turn-off thyristor in the controllable turn-off converter module can be short-circuited by opening the controllable turn-off thyristor in the bypass branch. The fault current is diverted through the bypass branch, so that the current at the controllable turn-off thyristor converter valve in the controllable turn-off converter module is 0, and the phase is naturally turned off, completing the commutation process. This solves the problem that the method of directly turning off the controllable turn-off converter valve cannot complete fault ride-through in the event of a severe asymmetrical fault in the receiving-end power grid.

[0061] 2) When a fault occurs in the AC system at the receiving end of the DC system, different fault ride-through strategies are adopted according to different fault degrees. That is, in the case of a minor fault, the commutation is assisted only by the controllable shut-off thyristor converter valve. In the case of a severe fault, the controllable shut-off thyristor converter valve is assisted by the bypass branch to indirectly assist the commutation. Therefore, on the one hand, it can adaptively ensure that the equipment is not affected by the peak overvoltage of the large current shutdown, and on the other hand, it can ensure that the converter valve operates stably and reliably during the fault.

[0062] 3) By mixing and connecting controllable turn-off thyristors and ordinary diodes in series on the bypass branch, the cost of implementing DC system fault ride-through method can be reduced while ensuring high current carrying capacity.

[0063] It should be understood that the specific embodiments described above are merely illustrative or explanatory of the principles of the invention and do not constitute a limitation thereof. Therefore, any modifications, equivalent substitutions, improvements, etc., made without departing from the spirit and scope of the invention should be included within the protection scope of the invention. Furthermore, the appended claims are intended to cover all variations and modifications falling within the scope and boundaries of the appended claims, or equivalent forms of such scope and boundaries.

Claims

1. A fault ride-through method for a DC system, characterized in that, The receiving end of the DC system is equipped with a controllable turn-off converter module. The controllable turn-off converter module includes a controllable turn-off thyristor converter valve and a bypass branch connected in parallel with the converter valve. Multiple controllable turn-off thyristors are connected in series on the bypass branch, and the controllable turn-off thyristors on the bypass branch are all in the same direction as the controllable turn-off thyristors in the controllable turn-off thyristor converter valve. The DC system fault ride-through method includes: real-time acquisition of the receiving-end grid side voltage and the receiving-end grid side zero-sequence voltage, and judgment on whether a fault has occurred in the AC system at the receiving end of the DC system and the degree of the fault. When a fault occurs in the AC system at the receiving end of the DC system, different fault ride-through strategies are adopted according to different fault severity. The specific methods are as follows: When a severe three-phase fault or asymmetrical fault occurs in the AC system at the receiving end, the controllable turn-off thyristor on the bypass branch is turned on to short-circuit the controllable turn-off thyristor converter valve in the controllable turn-off converter module, and the fault current is diverted through the bypass branch. After the controllable turn-off thyristor on the bypass branch is turned on, when the voltage amplitude of the receiving end grid side is greater than the second set threshold, the zero-sequence voltage of the receiving end grid is less than the third set threshold, and the deviation between the current DC current of the DC system and the DC current of the DC system under normal conditions is greater than the set deviation threshold, the controllable turn-off thyristor on the bypass branch is turned off, and the turn-off pulse of the controllable turn-off thyristor converter valve is enabled. When the voltage amplitude of the receiving-end grid is greater than the second set threshold, the zero-sequence voltage of the receiving-end grid is less than the third set threshold, and the deviation between the current DC current of the DC system and the DC current of the DC system under normal conditions is less than the set deviation threshold, the turn-off pulse of the controllable turn-off thyristor converter valve is not enabled after a set delay time.

2. The DC system fault ride-through method according to claim 1, characterized in that, The specific method for adopting different fault-crossing strategies based on different fault severity is as follows: When the voltage amplitude on the receiving end network side is greater than the first set threshold and less than the second set threshold, and the zero-sequence voltage on the receiving end network side is greater than the third set threshold and less than the fourth set threshold, it is determined that an AC low voltage fault has occurred in the AC system of the receiving end. The controllable turn-off thyristor on the bypass branch is kept off, and the turn-off pulse of the controllable turn-off thyristor converter valve is enabled to turn off the controllable turn-off thyristor converter valve in the controllable turn-off converter module. When the voltage amplitude on the receiving end grid side is less than the first set threshold, or the zero-sequence voltage on the receiving end grid side is greater than the fourth set threshold, it is determined that a serious three-phase fault or asymmetrical fault has occurred in the AC system at the receiving end. The first set threshold is less than the second set threshold; the third set threshold is less than the fourth set threshold.

3. The DC system fault ride-through method according to claim 1 or 2, characterized in that, A diode is also connected in series on the bypass branch; all diodes are in the same direction as the controllable turn-off thyristor on the bypass branch.

4. The DC system fault ride-through method according to claim 1, characterized in that, Within a set delay time, continuously determine whether the current system condition meets the following conditions: the voltage amplitude of the receiving end grid side is greater than the second set threshold, the zero-sequence voltage of the receiving end grid is less than the third set threshold, and the deviation between the current DC current of the DC system and the DC current of the DC system under normal conditions is less than the set deviation threshold.

5. The DC system fault ride-through method according to claim 2, characterized in that, When a serious three-phase fault or asymmetrical fault is detected in the AC system at the receiving end and the controllable turn-off thyristor on the bypass branch is turned on, the rectifier side is controlled to maintain a constant DC current operation.

6. A DC system fault ride-through system, characterized in that, The system includes a controllable turn-off converter module installed at the receiving end of a DC system. The controllable turn-off converter module includes a controllable turn-off thyristor converter valve and a bypass branch connected in parallel with the converter valve. Multiple controllable turn-off thyristors are connected in series on the bypass branch, and all controllable turn-off thyristors on the bypass branch are in the same direction as the controllable turn-off thyristors in the controllable turn-off thyristor converter valve. The DC system fault ride-through system also includes a processor for executing program instructions to implement the DC system fault ride-through method; The DC system fault ride-through method includes: real-time acquisition of the receiving-end grid side voltage and the receiving-end grid side zero-sequence voltage, and judgment on whether a fault has occurred in the AC system at the receiving end of the DC system and the degree of the fault. When a fault occurs in the AC system at the receiving end of the DC system, different fault ride-through strategies are adopted according to different fault severity. The specific methods are as follows: When a severe three-phase fault or asymmetrical fault occurs in the AC system at the receiving end, the controllable turn-off thyristor on the bypass branch is turned on to short-circuit the controllable turn-off thyristor converter valve in the controllable turn-off converter module, and the fault current is diverted through the bypass branch. After the controllable turn-off thyristor on the bypass branch is turned on, when the voltage amplitude of the receiving end grid side is greater than the second set threshold, the zero-sequence voltage of the receiving end grid is less than the third set threshold, and the deviation between the current DC current of the DC system and the DC current of the DC system under normal conditions is greater than the set deviation threshold, the controllable turn-off thyristor on the bypass branch is turned off, and the turn-off pulse of the controllable turn-off thyristor converter valve is enabled. When the voltage amplitude of the receiving-end grid is greater than the second set threshold, the zero-sequence voltage of the receiving-end grid is less than the third set threshold, and the deviation between the current DC current of the DC system and the DC current of the DC system under normal conditions is less than the set deviation threshold, the turn-off pulse of the controllable turn-off thyristor converter valve is not enabled after a set delay time.

7. The DC system fault ride-through system according to claim 6, characterized in that, The specific method for adopting different fault-crossing strategies based on different fault severity is as follows: When the voltage amplitude on the receiving end network side is greater than the first set threshold and less than the second set threshold, and the zero-sequence voltage on the receiving end network side is greater than the third set threshold and less than the fourth set threshold, it is determined that an AC low voltage fault has occurred in the AC system of the receiving end. The controllable turn-off thyristor on the bypass branch is kept off, and the turn-off pulse of the controllable turn-off thyristor converter valve is enabled to turn off the controllable turn-off thyristor converter valve in the controllable turn-off converter module. When the voltage amplitude on the receiving end grid side is less than the first set threshold, or the zero-sequence voltage on the receiving end grid side is greater than the fourth set threshold, it is determined that the AC system at the receiving end has a serious three-phase fault or an asymmetrical fault. The first set threshold is less than the second set threshold; the third set threshold is less than the fourth set threshold.

8. The DC system fault ride-through system according to claim 6 or 7, characterized in that, A diode is also connected in series on the bypass branch; all diodes are in the same direction as the controllable turn-off thyristor on the bypass branch.

9. The DC system fault ride-through system according to claim 6, characterized in that, Within a set delay time, continuously determine whether the current system condition meets the following conditions: the voltage amplitude of the receiving end grid side is greater than the second set threshold, the zero-sequence voltage of the receiving end grid is less than the third set threshold, and the deviation between the current DC current of the DC system and the DC current of the DC system under normal conditions is less than the set deviation threshold.

10. The DC system fault ride-through system according to claim 7, characterized in that, The processor is also used to control the rectifier side to maintain a constant DC current operation when it determines that a serious three-phase fault or asymmetrical fault has occurred in the AC system at the receiving end and turns on the controllable turn-off thyristor on the bypass branch.

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