Fault processing method and device, flexible direct current system and storage medium

Abstract

The invention discloses a fault processing method and device, a flexible direct current system and a storage medium, and relates to the technical field of system control. The method is applied to a flexible direct current system comprising a hybrid MMC, and comprises the following steps: when an MMC direct current fault occurs, obtaining a direct current side fault current and a fault voltage in real time; judging whether a first preset condition is met or not according to the fault current and the fault voltage; if yes, performing locking control on the MMC to realize fault isolation; otherwise, continuously judging whether a second preset condition is met according to the fault voltage; if yes, negative voltage control is carried out on the MMC, and fault ride-through is realized; otherwise, controlling the MMC to work normally; in the fault ride-through process, whether a third preset condition is met or not is judged according to the fault current; if yes, zero-voltage control is carried out on the MMC so as to recover normal work of the MMC; otherwise, performing locking control on the MMC to realize fault isolation. The invention provides a rapid and reliable flexible direct current fault processing method on the system level.

Description

Technical Field

[0001] The present application relates to the technical field of system control, and particularly to a fault handling method, device, flexible DC system, and storage medium. Background Technique

[0002] In a multi-terminal flexible DC power transmission system, MMC (Modular Multilevel Converter) is widely used. At present, the fault isolation scheme of a flexible DC power grid includes means such as clearing faults by blocking all converter stations or fault ride-through without blocking. However, there are problems that the fault clearing time is greatly affected, resulting in a long time-consuming, and the control process of fault ride-through is relatively complex. Summary of the Invention

[0003] The main purpose of the present application is to provide a fault handling method, device, flexible DC system, and storage medium, aiming to solve the technical problems that the fault isolation method of a flexible DC power transmission system in the related art is time-consuming and the control process is complex.

[0004] To achieve the above object, the present application proposes a fault handling method applied to a flexible DC system. The system includes a hybrid modular multilevel converter MMC. The fault handling method includes: When a DC fault occurs in the hybrid MMC, the fault current and fault voltage on the DC side of the hybrid MMC are obtained in real time; Judge whether the first preset condition is satisfied according to the fault current and the fault voltage; If the first preset condition is satisfied, the hybrid MMC is blocked to achieve fault isolation; otherwise, it is further judged whether the second preset condition is satisfied according to the fault voltage; If the second preset condition is satisfied, negative voltage control is performed on the hybrid MMC to achieve fault ride-through; otherwise, the hybrid MMC is controlled to operate normally; During the fault ride-through process, it is judged whether the third preset condition is satisfied according to the fault current; If the third preset condition is satisfied, zero voltage control is performed on the hybrid MMC to restore the normal operation of the hybrid MMC; otherwise, the hybrid MMC is blocked to achieve fault isolation.

[0005] In an embodiment, the hybrid MMC includes a converter circuit with a three-phase six-arm structure and a reactor. The reactor is located on the DC output bus of the converter circuit; Obtaining the fault current and fault voltage on the DC side of the hybrid MMC in real time includes: Obtaining the DC output bus voltage of the converter circuit; The fault current on the DC side of the hybrid MMC is obtained based on the first current calculation formula and the DC output bus voltage; wherein, the first current calculation formula is: , wherein, I dc is the fault current, U dc is the DC output bus voltage, s is the Laplace variable, R eq is the three-phase equivalent resistance of the commutation circuit, R f is the equivalent impedance of the DC fault point, L eq is the three-phase equivalent inductance of the commutation circuit, L dc is the reactance value of the reactor, C eq is the three-phase equivalent capacitance of the commutation circuit; The fault voltage on the DC side of the hybrid MMC is obtained based on the fault current and the equivalent impedance of the DC fault point.

[0006] In one embodiment, the fault current includes the DC positive current and the DC negative current, and the fault voltage includes the DC positive instantaneous voltage and the DC negative instantaneous voltage; The first preset condition is that the absolute value of the difference between the DC positive instantaneous voltage and the DC negative instantaneous voltage is less than or equal to the first voltage setting value, and the value of the DC positive current or the DC negative current is greater than or equal to the first current setting value; The second preset condition is that the absolute value of the sum of the DC positive instantaneous voltage and the DC negative instantaneous voltage is greater than or equal to the second voltage setting value.

[0007] In one embodiment, the hybrid MMC includes a commutation circuit with a three-phase six-arm structure and a reactor. The reactor is located on the DC output bus of the commutation circuit. Each arm of the commutation circuit includes a plurality of sub-modules. The plurality of sub-modules include at least one half-bridge sub-module and at least one full-bridge sub-module. Among them, the proportion of the full-bridge sub-modules in the plurality of sub-modules is greater than or equal to 50%; During the fault ride-through process, it is judged whether the third preset condition is satisfied according to the fault current, including: During the fault ride-through process, the DC output bus voltage of the commutation circuit is obtained; The fault current on the DC side of the hybrid MMC is obtained based on the second current calculation formula and the DC output bus voltage; wherein, the second current calculation formula is: , wherein, I dc is the fault current,h is the proportion of full-bridge sub-modules in each arm of the commutation circuit, h≥ 50%, U dc is the DC output bus voltage, s is the Laplace variable, L eq is the three-phase equivalent inductance of the commutation circuit, L dc is the reactance value of the reactor; Judge whether the third preset condition is satisfied according to the fault current.

[0008] In one embodiment, the fault current includes the DC positive current and the DC negative current; The third preset condition includes that the value of the DC positive current is less than or equal to the second current setting value, and the value of the DC negative current is less than or equal to the second current setting value. The second current setting value is determined based on the rated current of the hybrid MMC.

[0009] In one embodiment, the structures of the arms in the commutation circuit are the same. Each arm includes N sub-modules. The N sub-modules include at least one half-bridge sub-module and at least one full-bridge sub-module. Among them, the capacitance value of the single capacitor in the half-bridge sub-module is C 0h and the capacitance value of the single capacitor in the full-bridge sub-module is C 0f ; The method further includes: Obtain the three-phase equivalent resistance R arm according to the equivalent resistance calculation formula and the arm resistance of each arm R eq , and the equivalent resistance calculation formula is: , Obtain the three-phase equivalent inductance L arm according to the equivalent inductance calculation formula and the arm inductance of each arm L eq , and the equivalent inductance calculation formula is: , Obtain the three-phase equivalent capacitance C eq according to the equivalent capacitance calculation formula and the number of sub-modules N of each arm , wherein, ε is the equivalent capacitance coefficient, which is set corresponding to different stages of the DC fault, C 0 =C 0h +C0f 。

[0010] In one embodiment, zero-voltage control is performed on the hybrid MMC to restore the normal operation of the hybrid MMC, including: Performing zero-voltage control on the hybrid MMC and determining whether the DC fault is cleared; If it has been cleared, it is determined that the DC fault is a transient fault, and the hybrid MMC is controlled to operate normally; If it has not been cleared, it is determined that the DC fault is a permanent fault, and the hybrid MMC is controlled to disconnect the circuit breaker on the AC side and the disconnector on the DC side to achieve fault isolation.

[0011] To achieve the above object, the present application also proposes a fault processing device, including a memory, a processor, and a computer program stored on the memory and running on the processor. The computer program is configured to implement the steps of the fault processing method as described above.

[0012] In addition, to achieve the above object, the present application also proposes a flexible DC system, including a hybrid modular multilevel converter MMC and the fault processing device as described above that are connected to each other.

[0013] In addition, to achieve the above object, the present application also proposes a storage medium. The storage medium is a computer-readable storage medium, and a computer program is stored on the storage medium. When the computer program is executed by a processor, the steps of the fault processing method as described above are implemented.

[0014] One or more technical solutions proposed by the present application have at least the following technical effects: A fault handling method is proposed. When a DC fault occurs in a hybrid MMC in a flexible DC system, it is judged whether the first preset condition is satisfied according to the DC-side fault current and fault voltage obtained in real time. If the first preset condition is satisfied, the hybrid MMC is locked for control to achieve fault isolation, so that severe faults can be quickly and reliably removed, ensuring the safety of the power grid and improving the system reliability. If the first preset condition is not satisfied, it is continued to judge whether the second preset condition is satisfied according to the fault voltage obtained in real time. If the second preset condition is satisfied, negative voltage control is performed on the hybrid MMC to achieve fault ride-through. Then, during the fault ride-through process, it is judged whether the third preset condition is satisfied according to the fault current obtained in real time. If the third preset condition is satisfied, zero voltage control is performed on the hybrid MMC to restore the normal operation of the hybrid MMC, so that power supply can be quickly restored for instantaneous faults, minimizing the system power outage time and ensuring the voltage stability and power transmission of the power grid to the greatest extent. If the third preset condition is not satisfied, the hybrid MMC is locked for control to achieve fault isolation, so that the fault can be removed when the fault ride-through strategy fails, ensuring the safety of the power grid and further improving the system reliability. The method of this application provides a fast and reliable flexible DC fault handling method at the system level through a fault handling logic strategy combining multi-level judgment and control, and corresponding system operations are performed for different fault situations. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The accompanying drawings herein are incorporated into the specification and form a part of the specification, showing embodiments consistent with the present application, and are used together with the specification to explain the principles of the present application. In order to more clearly illustrate the embodiments of the present application or the technical solutions in the related art, the following will briefly introduce the drawings required to be used in the description of the embodiments or the related art. Obviously, for those of ordinary skill in the art, other drawings can be obtained according to these drawings without creative efforts.

[0016] Figure 1 It is a schematic flowchart of the first embodiment of the fault handling method provided by the present application; Figure 2 It is a schematic flowchart of the second embodiment of the fault handling method provided by the present application; Figure 3 It is an equivalent circuit model when a fault occurs in a hybrid MMC in the second embodiment of the fault handling method provided by the present application; Figure 4 It is an equivalent circuit model of a hybrid MMC during the fault ride-through process in the second embodiment of the fault handling method provided by the present application; Figure 5 is an equivalent circuit model based on Figure 4 and is an equivalent transformation to obtain a superimposed network equivalent model; Figure 6 It is a flowchart of an application example of the fault handling method provided by an embodiment of this application; Figure 7 It is a schematic diagram of the hardware structure of the fault handling device related to an embodiment of this application; Figure 8 It is a connection schematic diagram of the flexible DC system related to an embodiment of this application.

[0017] The realization of the purpose of this application, functional features and advantages will be further described in combination with embodiments with reference to the accompanying drawings. Specific Embodiments

[0018] It should be understood that the specific embodiments described herein are only used to explain the technical solution of this application and are not used to limit this application. To better understand the technical solution of this application, the following will be described in detail in combination with the specification drawings and specific embodiments.

[0019] Flexible DC is a high-voltage DC power transmission technology based on power electronics technology. In a multi-terminal flexible DC power transmission system, MMC (Modular Multilevel Converter) has been widely used in many fields such as long-distance large-capacity power transmission, asynchronous grid interconnection, and offshore wind power transmission due to its strong controllability, no need for reactive power compensation, and ability to supply power to passive networks. Compared with AC systems, the fault current in DC systems rises rapidly, has a high peak value, and lacks a natural zero-crossing point, making it difficult to isolate faults, which restricts the development and application of flexible DC power transmission technology.

[0020] Currently, there are mainly two fault isolation schemes for flexible DC grids: one is a half-bridge MMC and a DC circuit breaker. This scheme can selectively cut off the faulty line, but there are problems such as difficult development and high cost. The other is an MMC with fault self-clearing ability and a fast disconnector. This scheme can block the fault current, but it is necessary to block all converters in the DC grid, which will cause the interruption of power transmission, and it takes a long time for the converters to restart after the fault is cleared; although there is a method of fault ride-through without blocking in related technologies, specifically using sub-modules with negative level output ability to reduce the DC voltage output by the converter, and then limiting the short-circuit current to zero to ensure voltage stability and power transmission, this method also has problems that the fault clearing time is greatly affected by the controlled DC voltage, resulting in a long time-consuming, and the control process of fault ride-through is relatively complex.

[0021] It can be seen that the fault isolation methods in the related flexible DC power transmission systems have problems such as long time-consuming, complex control process, and high cost.

[0022] In view of the above problems, an embodiment of the present application proposes a fault handling method, device, flexible DC system, and storage medium. For ease of description, the fault handling device in the flexible DC system is used as the execution subject for detailed elaboration below.

[0023] It should be noted that the fault handling device is a computing service device with data processing, network communication, and program running functions, such as a server, network terminal, embedded computer, industrial control computer, etc. This fault handling device can be integrated with the control device of the flexible DC system. While implementing the fault handling method of the embodiment of the present application, it can also implement other control operations, such as the on / off control of switching devices, the output regulation of converters, the start / stop control of the system, etc., which are not specifically limited here.

[0024] An embodiment of the present application provides a fault handling method.

[0025] In the first embodiment of the fault handling method of the present application, refer to Figure 1 , Figure 1 is a schematic flow chart of the first embodiment of the fault handling method of the present application. This fault handling method can be applied to a flexible DC system, which can include a hybrid modular multilevel converter MMC, and a fault handling device or control device for implementing the fault handling method of this embodiment. This fault handling method can include steps S10 to S80: Step S10, when a DC fault occurs in the hybrid MMC, obtain the fault current and fault voltage on the DC side of the hybrid MMC in real time.

[0026] It should be noted that the hybrid MMC refers to a modular multilevel converter in which each arm contains two or more different structured sub-modules, such as a combination of a half-bridge sub-module (HBSM: Half Bridge Sub-Module) and a full-bridge sub-module (FBSM: Full Bridge Sub-Module). A DC fault refers to a short-circuit fault that occurs on the DC side of the hybrid MMC, such as an inter-pole short circuit or a single-pole grounding fault. The fault current may rise rapidly during a fault, which may cause damage to system equipment. The fault current refers to the current flowing through the fault point when a DC fault occurs in the hybrid MMC, and the fault voltage refers to the instantaneous change of the DC side voltage when a DC fault occurs in the hybrid MMC.

[0027] It should also be noted that the fault current can include the positive and negative currents on the DC side of the hybrid MMC. The real-time acquisition of the fault current can be obtained by calculating the DC bus voltage of the hybrid MMC in real time. The fault current can also include the current at the fault point, which can be detected by a separately provided current detection module or circuit. The fault voltage can include the instantaneous positive and negative voltages on the DC side of the hybrid MMC. The real-time acquisition of the fault voltage can be obtained by further calculation based on the calculated fault current. The fault voltage can also include the voltage at the fault point, which can be detected by a separately provided voltage detection module or circuit. In this embodiment, the specific data types and acquisition methods of the fault current and fault voltage are not specifically limited.

[0028] Step S20: Determine whether the first preset condition is satisfied according to the fault current and the fault voltage.

[0029] It should be noted that the first preset condition is set based on the fault current and the fault voltage. It can be a direct threshold range requirement for both the fault current and the fault voltage. Correspondingly, satisfying the first preset condition can be directly requiring that the fault current and the fault voltage both satisfy their respective preset threshold ranges. That is, when the fault current satisfies the current threshold range in the first preset condition and the fault voltage also satisfies the voltage threshold range in the first preset condition, it is considered to satisfy the first preset condition. It can also be a threshold range requirement indirectly related to the fault current and the fault voltage. Correspondingly, satisfying the first preset condition can be requiring that the indirect values obtained from the fault current (such as mean value, sum value, difference value, absolute value, etc.) and the indirect values obtained from the fault voltage (such as mean value, sum value, difference value, absolute value, etc.) both satisfy their respective preset threshold ranges. That is, when the indirect value obtained from the fault current satisfies the threshold range in the first preset condition and the indirect value obtained from the fault voltage also satisfies the threshold range in the first preset condition, it is considered to satisfy the first preset condition. Among them, the preset threshold range can be set according to actual needs. For example, the current threshold range or voltage threshold range can be set corresponding to the voltage and current shown by the hybrid MMC during a severe fault. If it is not within this range, it does not belong to a severe fault, or if one of them is not within the corresponding preset threshold range, it also does not belong to a severe fault. It can also be calculated and set the corresponding threshold range according to the voltage and current shown by the hybrid MMC during a severe fault. In practical applications, the first preset condition can be set according to actual needs, and no specific limitation is made here.

[0030] Step S30: If the first preset condition is satisfied, perform a blocking control on the hybrid MMC to achieve fault isolation.

[0031] It should be noted that fault isolation refers to quickly identifying and isolating the fault area after detecting a fault to prevent the fault from spreading to the entire system. Lockout control refers to a control method that shuts off the power devices of all sub-modules in the hybrid MMC, forcibly bypasses the capacitor devices of the sub-modules, and stops the operation of the hybrid MMC and isolates the fault current.

[0032] If it is determined that the current hybrid MMC meets the first preset condition based on the fault current and fault voltage, it indicates that the DC fault of the hybrid MMC is a serious fault such as an inter-pole short circuit fault. If a fault ride-through is performed, it may cause more equipment failures. Therefore, at this time, a control strategy of locking the converter can be adopted to lock the hybrid MMC to cut off the fault and achieve fault isolation.

[0033] Step S40, if the first preset condition is not met, then further determine whether the second preset condition is met based on the fault voltage.

[0034] It should be noted that if it is determined that the current hybrid MMC does not meet the first preset condition based on the fault current and fault voltage, it indicates that the DC fault of the hybrid MMC is not a serious fault. At this time, subsequent operations can be continued, and specifically, further determine whether the second preset condition is met based on the fault voltage obtained in real time.

[0035] It should also be noted that the second preset condition is set separately based on the fault voltage. It can be a direct threshold range requirement for the fault voltage, and meeting the second preset condition means that the fault voltage meets the corresponding preset threshold range; it can also be a threshold range requirement indirectly related to the fault voltage, and meeting the second preset condition means that the indirect value (such as mean value, sum value, difference value, absolute value, etc.) obtained based on the fault voltage meets the corresponding preset threshold range. Among them, the preset threshold range can be set according to actual needs. For example, it can be set corresponding to the voltage shown by the hybrid MMC in some general faults or instantaneous faults where fault ride-through can be performed. If it is within this range, it indicates that the current DC fault is not a serious fault but a fault where fault ride-through can be performed. If it is not within this range, it indicates that the current DC fault is neither a serious fault nor a fault that requires fault ride-through, and may just be a small fluctuation during normal operation, and continuing normal operation will not affect the system or internal equipment; it can also be calculated and set corresponding threshold ranges according to the voltage shown by the hybrid MMC in general faults or instantaneous faults where fault ride-through can be performed. In practical applications, the second preset condition can be set according to actual needs, and no specific limitation is made here.

[0036] Step S50, if the second preset condition is met, then perform negative voltage control on the hybrid MMC to achieve fault ride-through.

[0037] It should be noted that fault ride through (FRT) refers to the ability of the system to maintain operation and continue to transmit power through control strategy adjustment when a fault occurs, avoiding system collapse or large-scale power outages caused by faults. Compared with fault isolation, fault ride through focuses on operating with faults, while fault isolation focuses on quickly removing faults. Negative voltage control refers to a DC voltage control mode that actively reduces the DC side voltage by controlling the hybrid MMC to output a negative level (such as discharging the capacitors of each sub-module), and reduces the voltage below zero to match the fault point voltage or suppress overcurrent.

[0038] If it is determined according to the fault voltage that the current hybrid MMC meets the second preset condition, it indicates that the DC fault of the hybrid MMC is not a serious fault, but a general fault or an instantaneous fault, etc., which can perform fault ride through, such as a single-pole grounding fault. If the converter is directly blocked, it will cause the power transmission to be interrupted, and it will also take a long time to restart the converter later. Therefore, at this time, a negative DC voltage control strategy can be adopted to perform negative voltage control on the hybrid MMC to achieve buck and current limiting without blocking and realize fault ride through.

[0039] Step S60, if the second preset condition is not met, control the hybrid MMC to operate normally.

[0040] It should be noted that if it is determined according to the fault voltage that the current hybrid MMC does not meet the second preset condition, it indicates that the DC fault of the hybrid MMC is not a serious fault, nor a fault that requires fault ride through. It may be considered that this DC fault is just a small fluctuation. At this time, the normal control of the hybrid MMC can be maintained to make the hybrid MMC continue to operate normally.

[0041] Step S70, during the fault ride through process, judge whether the third preset condition is met according to the fault current.

[0042] It should be noted that fault ride-through has a current-limiting effect. During this period, step S10 will continue to obtain the fault current in real time. The third preset condition is set separately based on the fault current. It can be a direct threshold range requirement for the fault current, and meeting the third preset condition means that the fault current meets the corresponding preset threshold range; it can also be a threshold range requirement indirectly related to the fault current, and meeting the third preset condition means that the indirect value (such as mean value, sum value, difference value, absolute value, etc.) obtained from the fault current meets the corresponding preset threshold range. Among them, the preset threshold range can be set according to actual needs. For example, the threshold range can be set corresponding to the current shown by the hybrid MMC during normal operation or small fluctuations. If it is within this range, it means that the current DC fault does not belong to a serious fault, nor is it a fault that can perform fault ride-through. The current fault current has returned to the normal range that will not affect the internal equipment of the system, and the normal operation of the system can be restored subsequently; if it is not within this range, it means that although the current DC fault can briefly maintain power transmission through fault ride-through, fault ride-through is not a long-term solution because the fault current has not been effectively reduced or restored to the normal range during fault ride-through, indicating that the current DC fault does not belong to a serious fault but belongs to a permanent fault, or indicating that the current-limiting of the previously executed fault ride-through has failed, and further fault response measures should be taken subsequently; it can also be calculated and set the corresponding threshold range according to the current shown by the hybrid MMC during normal operation or small fluctuations. In practical applications, the second preset condition can be set according to actual needs, and no specific limitation is made here.

[0043] Step S80, if the third preset condition is met, perform zero-voltage control on the hybrid MMC to restore the normal operation of the hybrid MMC; If the third preset condition is not met, perform blocking control on the hybrid MMC to achieve fault isolation.

[0044] It should be noted that zero-voltage control refers to controlling the DC voltage output by the hybrid MMC to zero, that is, by controlling the switching devices of all sub-modules, making the capacitors of the sub-modules neither charge nor discharge, so as to make the output voltage zero DC voltage control mode. During the specific implementation process, as Figure 1 shown, if the judgment result of step S70 is negative, it can directly jump to step S30 to execute the step of performing blocking control on the hybrid MMC to achieve fault isolation.

[0045] If it is determined according to the fault current that the current hybrid MMC meets the third preset condition, it indicates that for the DC fault of the hybrid MMC, the fault current has been successfully restored to the normal state through the current-limiting effect of fault ride-through. To shorten the fault recovery time, once it is found that the third preset condition is met during fault ride-through, the control strategy for the hybrid MMC can be immediately switched, and the zero DC voltage control strategy is adopted to perform zero-voltage control on the hybrid MMC to avoid reverse current impact, so as to restore the normal operation of the hybrid MMC. After the hybrid MMC establishes the DC voltage, the active power transmission of the line is restored, thereby enabling the entire system to resume normal operation.

[0046] If it is determined according to the fault current that the current hybrid MMC does not meet the third preset condition, it indicates that for the DC fault of the hybrid MMC, the fault current has not been restored to the normal state through the current-limiting effect of fault ride-through. This may be because the DC fault is a permanent fault or the current-limiting function of the fault ride-through fails. At this time, the control strategy of blocking the converter can be adopted to perform blocking control on the hybrid MMC to completely cut off the fault and achieve fault isolation. After the subsequent fault is solved, if power supply needs to be restored again, the hybrid MMC is restarted.

[0047] It can be understood that in the method of this embodiment, considering the multiple types of overhead line faults in the flexible DC system, large influence by the environment, and mostly single-pole grounding faults and other situations, the non-blocking fault ride-through strategy is used to perform fault ride-through when it is determined that the first preset condition (not a serious fault) is not met but the second preset condition is met, which can ensure the voltage stability and power transmission of the backend DC grid to the greatest extent and can quickly restore power supply; in this method, for serious faults or when the non-blocking fault ride-through strategy fails (a permanent fault), the blocking control strategy is used for fault isolation, which can reliably cut off the fault and ensure the safety of the backend DC grid.

[0048] This embodiment provides a fault handling method. When a DC fault occurs in a hybrid MMC in a flexible DC system, it is determined whether the first preset condition is met based on the DC-side fault current and fault voltage obtained in real time. If the first preset condition is met, the hybrid MMC is locked to achieve fault isolation, enabling fast and reliable removal of serious faults, ensuring grid safety, and improving system reliability. If the first preset condition is not met, it is then determined whether the second preset condition is met based on the fault voltage obtained in real time. If the second preset condition is met, negative voltage control is performed on the hybrid MMC to achieve fault ride-through. Then, during the fault ride-through process, it is determined whether the third preset condition is met based on the fault current obtained in real time. If the third preset condition is met, zero voltage control is performed on the hybrid MMC to restore its normal operation, enabling fast power restoration for transient faults, minimizing system power outage time, and maximizing grid voltage stability and power transmission. If the third preset condition is not met, the hybrid MMC is locked to achieve fault isolation, enabling fault removal when the fault ride-through strategy fails, ensuring grid safety, and further improving system reliability. The method of this application provides a fast and reliable flexible DC fault handling method at the system level through a fault handling logic strategy that combines multi-level judgment and control and performs corresponding system operations for different fault conditions.

[0049] In a feasible implementation, as Figure 1 shown, "locking the hybrid MMC to achieve fault isolation" in step S30 may include step S31: Step S31, lock the hybrid MMC and disconnect the circuit breaker on the AC side and the disconnector on the DC side of the hybrid MMC to achieve fault isolation.

[0050] It should be noted that in the hybrid MMC of the flexible DC system, in addition to the commutation circuit that plays a key and main role, a circuit breaker can be provided on its AC side to isolate the AC power supply from the commutation circuit, and a disconnector can be provided on its DC side to isolate the commutation circuit and the power grid. To ensure complete isolation of the fault by the hybrid MMC and the overall reliability and safety of the system, while performing locking control on the switching devices of each arm in the commutation circuit, the circuit breaker on the AC side and the disconnector on the DC side can be disconnected to prevent damage to other devices before and after the hybrid MMC caused by this fault.

[0051] In this implementation, while locking the hybrid MMC, the circuit breaker on the AC side and the disconnector on the DC side of the hybrid MMC can be disconnected to prevent damage to the hybrid MMC or the devices before and after it caused by the DC fault, avoid the expansion of the impact caused by the fault, and prevent adverse effects on the system operation and recovery after the fault is removed.

[0052] Based on the first embodiment of the fault handling method of the present application, in the second embodiment of the fault handling method of the present application, for the content that is the same as or similar to the above embodiment, reference can be made to the above introduction and will not be elaborated hereinafter. On this basis, referring to Figure 2 , Figure 2 is a schematic flowchart of the second embodiment of the fault handling method of the present application. Step S80 may include steps S81 to S84: Step S81, perform zero-voltage control on the hybrid MMC; Step S82, determine whether the DC fault is cleared; Step S83, if it has been cleared, determine that the DC fault is a transient fault, and control the hybrid MMC to operate normally; Step S84, if it has not been cleared, determine that the DC fault is a permanent fault, and disconnect the circuit breaker on the AC side and the disconnector on the DC side of the hybrid MMC to achieve fault isolation.

[0053] It should be noted that during the fault ride-through process, if it is determined according to the fault current that the third preset condition is met, the DC voltage control mode of the hybrid MMC can be switched. Specifically, the negative voltage control is switched to zero-voltage control. At the same time, a system judgment process can be added to specifically judge whether the DC fault is cleared. Specifically, it can be judged based on the output result of a fault detection device or the like in the system. If the DC fault has been cleared, it means that the DC fault is a transient fault. At this time, the hybrid MMC can be normally controlled to restore the system to normal operation. If the DC fault has not been cleared, it means that the DC fault is a permanent fault. At this time, the circuit breaker and disconnector in the hybrid MMC can be disconnected to achieve fault isolation. For more specific implementation manners, refer to the description of the foregoing embodiments and will not be elaborated herein.

[0054] It can be understood that by adding a judgment process corresponding to fault clearing, it can be clearly known whether the DC fault is a transient fault or a permanent fault, so as to further perform system restoration or fault isolation. It can not only intuitively and clearly present the fault handling process, but also further ensure the reliability and safety of the system.

[0055] In a feasible implementation manner, the hybrid MMC may include a commutation circuit with a three-phase six-arm structure and a reactor. The reactor is located on the DC output bus of the commutation circuit. The reactor may be specifically located on the positive DC bus or the negative DC bus of the DC output, and no specific limitation is made here.

[0056] Correspondingly, "obtain the fault current and fault voltage on the DC side of the hybrid MMC in real time" in step S10 may include steps S11 to S13: Step S11, obtain the DC output bus voltage of the commutation circuit U dc ; Step S12, obtain the fault current on the DC side of the hybrid MMC according to the first current calculation formula and the DC output bus voltage U dc ; where the first current calculation formula is: , wherein, I dc is the fault current, U dc is the DC output bus voltage, s is the Laplace variable, R eq is the three-phase equivalent resistance of the commutation circuit, R f is the equivalent impedance of the DC fault point, L eq is the three-phase equivalent inductance of the commutation circuit, L dc is the reactance value of the reactor, C eq is the three-phase equivalent capacitance of the commutation circuit; Step S13, obtain the fault voltage on the DC side of the hybrid MMC according to the fault current and the equivalent impedance of the DC fault point R f ;

[0057] It should be noted that the discharge path formed after the hybrid MMC has a DC fault is the main channel for the energy release of the flexible DC system, which directly affects the magnitude of the DC short-circuit current. Since during the fault handling process of this hybrid MMC, it is necessary to go through two judgment processes before deciding whether to perform fault ride-through, that is, the DC control strategy of this hybrid MMC needs to be switched to the fault ride-through mode after a period of time. Therefore, when this hybrid MMC has a DC fault, whether it is a severe fault such as an inter-pole short-circuit fault or a general fault such as a single-pole grounding fault, this hybrid MMC can be equivalently regarded as an RLC discharge circuit with initial capacitor energy storage, as Figure 3 shown is the equivalent circuit model of the hybrid MMC when a fault occurs in this embodiment.

[0058] As Figure 3 shown, after detecting that the hybrid MMC has a DC fault and before performing fault ride-through, the equivalent model of the hybrid MMC includes a three-phase equivalent inductance L eq , a three-phase equivalent resistance R eq , and a three-phase equivalent capacitance Ceq , reactor L dc , equivalent impedance of DC fault point R f The circuit formed. In this case, the fault current obtained in real time can be calculated according to the DC output bus voltage of the commutation circuit U dc and the above first current calculation formula, and then the fault voltage is calculated to perform the judgment of step S20 or S40.

[0059] In an alternative embodiment, the structures of the arms in the commutation circuit are the same. Each arm includes N sub-modules, and the N sub-modules include at least one half-bridge sub-module and at least one full-bridge sub-module. Among them, the capacitance value of the single capacitor in the half-bridge sub-module is C 0h , and the capacitance value of the single capacitor in the full-bridge sub-module is C 0f .

[0060] Correspondingly, the fault handling method may further include steps S01 to S03: Step S01, obtaining the three-phase equivalent resistance R arm according to the equivalent resistance calculation formula and the arm resistance of each arm R eq , and the equivalent resistance calculation formula is: , Step S02, obtaining the three-phase equivalent inductance L arm according to the equivalent inductance calculation formula and the arm inductance of each arm L eq , and the equivalent inductance calculation formula is: , Step S03, obtaining the three-phase equivalent capacitance C eq according to the equivalent capacitance calculation formula and the number N of sub-modules of each arm , wherein, ε is the equivalent capacitance coefficient, which is set corresponding to different stages of the DC fault, C 0 =C 0h +C 0f .

[0061] It should be noted that the above steps S01 to S03 can be executed before step S10, or can be accompanied by the fault handling process. When it is necessary to obtain the fault current and fault voltage in real time before each judgment of the preset conditions, it is first executed to determine the three-phase equivalent resistance of the commutation circuit R eq , the three-phase equivalent inductance L eq and the three-phase equivalent capacitance C eq After that, the fault current can be quickly calculated.

[0062] Furthermore, the fault current may include the DC positive current I dP and the DC negative current I dN , and the fault voltage may include the DC positive instantaneous voltage U dP and the DC negative instantaneous voltage U dN .

[0063] In a specific embodiment, the first preset condition may be that the absolute value of the difference between the DC positive instantaneous voltage U dP and the DC negative instantaneous voltage U dN is less than or equal to the first voltage setting value U d.set1 , and the DC positive current U dP or the DC negative current U dN is greater than or equal to the first current setting value I d.set1 .

[0064] It should be noted that based on the foregoing specific description, when the first preset condition is satisfied, it indicates that a serious fault has occurred in the hybrid MMC. Therefore, the first preset condition can be set based on specific serious fault conditions. Exemplarily, the first preset condition is set based on the pole-to-pole short-circuit fault to ensure that the blocking control of the hybrid MMC can act reliably when the pole-to-pole short-circuit occurs. The pole-to-pole short-circuit fault here refers to an abnormal conduction between the two poles (positive and negative) on the DC side, resulting in current flowing directly from one pole to the other pole instead of through the normal load path. This kind of fault may be caused by insulation failure, equipment breakdown, external environmental damage (such as lightning strike or foreign object intrusion), or system abnormality.

[0065] Among them, the first preset condition may be , where && represents logical AND, and || represents logical OR. Step S20 may include determining whether the current satisfies , if satisfied, execute step S30; otherwise, execute step S40.

[0066] In another specific embodiment, the second preset condition may be the absolute value of the sum of the instantaneous DC positive voltage U dP and the instantaneous DC negative voltage U dN is greater than or equal to the second voltage setting value U d.set2 .

[0067] It should be noted that based on the foregoing specific description, when the second preset condition is satisfied, fault ride-through needs to be executed. Therefore, the second preset condition can be set corresponding to the general faults or instantaneous fault conditions that can specifically execute fault ride-through. Exemplarily, the second preset condition is set based on a single-pole grounding fault to ensure reliable operation of the fault ride-through of the hybrid MMC when a single-pole grounding fault occurs. The single-pole grounding fault here refers to an abnormal conduction between a certain pole (positive or negative) on the DC side and the ground (earth), resulting in current leakage through the grounding path. This kind of fault is usually caused by equipment insulation defects, external force damage (such as construction accidents), cable aging or lightning strikes, etc. It is one of the more common and harmful fault types in the flexible DC system.

[0068] Specifically, the second preset condition is . Step S40 may include determining whether the current satisfies , if satisfied, execute step S50; otherwise, execute step S60.

[0069] The fault handling method provided in this embodiment, based on the electrical quantity characteristics obtained in real time after the fault, such as the instantaneous voltage and the positive and negative pole currents, sets the first preset condition and the second preset condition correspondingly, and then dynamically matches different control strategies, realizing the fast blocking and fault isolation of severe faults, as well as the fault ride-through and fast recovery of instantaneous faults, which can ensure the safe and stable operation of the backend power grid, minimize the power outage time to the greatest extent, improve the power supply reliability and power supply duration of the system.

[0070] In another feasible embodiment, the hybrid MMC may include a commutation circuit with a three-phase six-bridge-arm structure and a reactor. The reactor is located on the DC output bus of the commutation circuit. Each bridge arm in the commutation circuit includes a plurality of sub-modules, and the plurality of sub-modules include at least one half-bridge sub-module and at least one full-bridge sub-module, where the proportion of the full-bridge sub-modules in the plurality of sub-modules is greater than or equal to 50%.

[0071] Among them, the structures of each bridge arm may be the same, that is, the total number of sub-modules of each bridge arm may be the same, and the proportion of the half-bridge sub-modules and the full-bridge sub-modules may also be the same. Exemplarily, each bridge arm of the hybrid MMC may be composed ofN HB Half - bridge sub - modules (HBSMs), N FB and full - bridge sub - modules (FBSMs). The number of sub - modules in each arm N = N HB +N FB .

[0072] Correspondingly, step S70 may include steps S71 - S73: Step S71, during the fault - ride - through process, obtain the DC output bus voltage of the commutation circuit; Step S72, obtain the fault current on the DC side of the hybrid MMC according to the second current calculation formula and the DC output bus voltage; where the second current calculation formula is: , where, I dc is the fault current, h is the proportion of full - bridge sub - modules in each arm of the commutation circuit, h≥ 50%, U dc is the DC output bus voltage, s is the Laplace variable, L eq is the three - phase equivalent inductance of the commutation circuit, L dc is the reactance value of the reactor; Step S73, judge whether the third preset condition is satisfied according to the fault current.

[0073] It should be noted that during the DC fault - ride - through of the hybrid MMC, the full - bridge sub - modules in each arm of the hybrid MMC will be in a negative - input state, causing the DC side to output a negative voltage. Taking the non - blocking DC fault - ride - through as an example, the DC reference voltage of each arm is set to the lower limit of the voltage adjustable range , if a negative DC voltage is to be output, the proportion of full - bridge sub - modules should be greater than 50%. Therefore, this fault - handling method is more applicable to hybrid MMCs with a full - bridge sub - module proportion h≥ of 50%. Based on the foregoing example, this full - bridge sub - module proportion h is: .

[0074] Based on this, during the DC fault - ride - through, the hybrid MMC can suppress the DC short - circuit current by actively outputting negative levels of sub - modules, essentially changing the equivalent circuit model when the hybrid MMC fails. As Figure 4 shown is the equivalent circuit model of the hybrid MMC during the fault - ride - through process in this embodiment.

[0075] As shown Figure 4 in the figure, during the fault ride-through process, the equivalent model of the hybrid MMC includes three-phase equivalent inductors connected in sequence L eq , three-phase equivalent resistors R eq , three-phase equivalent capacitors C eq , an equivalent negative voltage source , a reactor L dc , and the equivalent impedance of the DC fault point R f to form a loop. According to the superposition theorem, the change in the circuit structure caused during the DC fault ride-through can be equivalently represented as the superposition of an initial network before the DC fault ride-through and an additional network during the DC fault ride-through, as Figure 5 shown in the figure Figure 4 is the equivalent circuit model based on

[0076] As shown Figure 5 in the figure, the left side is the initial network, and the right side is the additional network. The "+" between them represents the network superposition. Due to the high-frequency characteristics of the impedance, the effects of the resistor and capacitor can be ignored during the DC fault ride-through process, and the fault current expression for current limiting and voltage reduction suppression of the branch during the fault ride-through process can be obtained, that is, the second current calculation formula: ; In this case, the fault current obtained in real time can be calculated based on the DC output bus voltage U dc of the commutation circuit and the above second current calculation formula, and then the judgment in step S70 can be performed according to the fault current.

[0077] In this embodiment, the fault current is calculated based on the network superposition during the DC fault ride-through. There is no need to perform complex switching timing conversion on the initial network, and only the calculation result of the additional network needs to be superimposed on the initial network at the switching moment. The current calculation in different stages after the fault is simplified through the superposition algorithm, significantly improving the calculation efficiency while meeting the engineering accuracy requirements, making this method applicable to the rapid analysis and protection setting of power grid faults. Moreover, when there are multiple converters in the system for DC fault ride-through, it can be equivalently represented as the superposition of the additional networks of multiple DC faults, and the above method of calculating the fault current based on the network superposition during the DC fault ride-through is still applicable.

[0078] Furthermore, the fault current can include the DC positive current I dP and the DC negative current IdN 。

[0079] In a specific embodiment, the third preset condition may include that the value of the DC positive current I dP is less than or equal to the second current setting value I d.set2 , and the value of the DC negative current I dN is less than or equal to the second current setting value I d.set2 . The second current setting value I d.set2 is determined based on the rated current of the hybrid MMC.

[0080] It should be noted that based on the foregoing specific description, when the third preset condition is satisfied, the normal operation of the hybrid MMC needs to be restored. Therefore, the third preset condition can be set by combining the error range and the rated current. Exemplarily, the third preset condition is based on error tuning to ensure that when the fault current changes within a certain error range, the normal operation of the hybrid MMC can be restored.

[0081] Specifically, the third preset condition can be , where , I n represents the rated current, k is a preset coefficient determined according to the possible errors in actual implementation and calculation. Step S70 may include determining whether the current satisfies . If it is satisfied, step S80 is executed; otherwise, step S30 is executed.

[0082] The fault handling method provided in this embodiment simplifies the calculation of the fault current within the allowable range of engineering tuning error, avoiding the complex modeling and time-consuming calculation using electromagnetic transient simulation software.

[0083] Exemplarily, to facilitate understanding of the implementation process of the fault handling method of the embodiments of the present application, refer to Figure 6 , Figure 6 A flowchart of an application example of a fault handling method is provided. Specifically: For the control strategy of the flexible DC transmission system of the multi-terminal hybrid MMC, the non-blocking fault ride-through strategy can quickly clear the fault current under the controllable state of the converter. However, this strategy is not foolproof and there is still a possibility of failure. As the most direct and reliable fault clearing means, the blocking control strategy can serve as its backup protection scheme to ensure that the converter can safely and reliably ride through DC faults. In view of this, a hybrid fault handling scheme that combines the non-blocking and blocking strategies is proposed. By the characteristics of fault electrical quantities, the fault types are distinguished and appropriate fault strategies are adopted, so as to improve the safety and rapidity during the fault ride-through process, maximize the power transmission of the DC grid, and enhance its ability to resist fault disturbances.

[0084] As Figure 6 shown, first, it is judged whether the criterion 1 is satisfied. When the MMC DC side outlet current reaches the criterion 1, it indicates that a serious fault has occurred. Therefore, the control strategy of blocking the converter is adopted to cut off the fault. If the criterion 1 is not satisfied, the next step is skipped. The criterion 1 here is set based on the pole-to-pole short-circuit fault, specifically ; Then, it is further judged whether the criterion 2 is satisfied. When the MMC DC side outlet current of a certain MMC reaches the criterion 2, the non-blocking fault ride-through strategy is adopted to step down and limit the current of this MMC based on the negative DC voltage control to achieve fault ride-through. If the criterion 2 is not satisfied, it is considered as a small fluctuation and it can continue to operate normally or directly end the fault handling process. The criterion 2 here is set based on the single-pole grounding fault, specifically ; During the fault ride-through process, that is, after the fault current is limited, a certain time delay can be set and then the subsequent judgment is executed; Continuously, it is judged whether the criterion 3 is satisfied. When the MMC DC side outlet current of a certain MMC reaches the criterion 3, the negative DC voltage control is switched to zero DC voltage control to avoid the reverse current impact. At this time, the MMC switches to the normal working condition and the MMC establishes the DC voltage. If the criterion 3 is not satisfied, it is considered that the DC fault is a permanent fault or the fault ride-through current limiting fails, and the control strategy of blocking the converter can be adopted to achieve fault isolation. The criterion 3 here is set based on the error, specifically , ; Then, it can be judged whether the fault is cleared. When the system recognizes that the fault is cleared, it indicates that the non-blocking current limiting is successful and the DC fault is a transient fault. Each MMC resumes the normal control state, the system resumes the active power transmission of the line, and the system resumes normal operation. When the system recognizes that the fault is not cleared, it indicates that the DC fault is a permanent fault. At this time, the fault line can be disconnected under the zero current condition. Specifically, the circuit breaker on the AC side of the MMC and the disconnector on the DC side can be disconnected to isolate the fault. If power supply needs to be restored later, each MMC needs to be restarted.

[0085] In this specific application example, the current calculation method for different stages of current after a multi-terminal DC system fault is simplified through a superposition algorithm, which significantly improves the calculation efficiency while meeting the engineering accuracy requirements. The proposed method is applicable to the rapid analysis of power grid faults and protection setting; based on electrical quantity characteristics such as voltage and current after a fault, different control strategies are dynamically matched to achieve the rapid blocking and isolation of severe faults, as well as the rapid recovery of instantaneous faults, improving the reliability and safety of the flexible DC system.

[0086] It should be noted that the above examples are only for understanding this application and do not constitute a limitation on the fault handling method of this application. Any simple transformation in more forms based on this technical concept is within the protection scope of this application.

[0087] This application also provides a fault handling device, which may include: at least one processor; and a memory communicatively connected to the at least one processor; wherein, the memory stores instructions executable by the at least one processor, and the instructions are executed by the at least one processor to enable the at least one processor to execute the fault handling method in the above embodiments.

[0088] Next, referring to Figure 7 , Figure 7 shows a schematic hardware structure diagram of a fault handling device suitable for implementing the embodiments of this application. The fault handling device may include, but is not limited to, fixed terminals such as servers, network terminals, embedded computers, industrial control computers, desktop computers, PCs (Personal Computers), digital TVs, and so on. Figure 7 The shown fault handling device is only an example and should not bring any limitation to the functions and usage scope of the embodiments of this application.

[0089] Such as Figure 7As shown, the fault handling device may include a processing device 1001 (such as a central processing unit, a graphics processing unit, etc.), which may perform various appropriate actions and processes according to a program stored in a read-only memory (ROM: Read Only Memory) 1002 or a program loaded from a storage device 1003 into a random access memory (RAM: Random Access Memory) 1004. In the RAM 1004, various programs and data required for the operation of the fault handling device are also stored. The processing device 1001, the ROM 1002, and the RAM 1004 are connected to each other via a bus 1005. An input / output (I / O) interface 1006 is also connected to the bus 1005. Generally, the following systems may be connected to the I / O interface 1006: an input device 1007 including, for example, a touch screen, a touchpad, a keyboard, a mouse, an image sensor, a microphone, an accelerometer, a gyroscope, etc.; an output device 1008 including, for example, a liquid crystal display (LCD: Liquid Crystal Display), a speaker, a vibrator, etc.; a storage device 1003 including, for example, a magnetic tape, a hard disk, etc.; and a communication device 1009. The communication device 1009 may allow the fault handling device to communicate with other devices wirelessly or wiredly to exchange data. Although the figure shows a fault handling device having various systems, it should be understood that it is not required to implement or have all the shown systems, and instead, more or fewer systems may be implemented or had.

[0090] In particular, according to the embodiments disclosed in the present application, the processes described above with reference to the flowcharts may be implemented as computer software programs. For example, the embodiments disclosed in the present application include a computer program product, which includes a computer program carried on a computer-readable storage medium, and the computer program contains program codes for performing the methods shown in the flowcharts. In such an embodiment, the computer program may be downloaded and installed from a network via the communication device 1009, or installed from the storage device 1003, or installed from the ROM 1002. When the computer program is executed by the processing device 1001, the above-mentioned functions defined in the fault handling method disclosed in the embodiments of the present application are executed.

[0091] The fault handling device provided by the present application, adopting the fault handling method in the above embodiment, can solve the technical problems that the fault isolation method in the related flexible DC power transmission system is time-consuming and the control process is complex. Compared with the related art, the beneficial effects of the fault handling device provided by the present application are the same as those of the fault handling method provided by the above embodiment, and other technical features in the fault handling device are the same as those disclosed in the fault handling method of the above embodiment, and will not be elaborated herein.

[0092] It should be understood that the various parts disclosed in this application can be implemented by hardware, software, firmware, or a combination thereof. In the description of the above embodiments, specific features, structures, materials, or characteristics can be combined in a suitable manner in any one or more embodiments or examples.

[0093] The above is only the specific implementation manner of this application, but the protection scope of this application is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed in this application should be covered by the protection scope of this application. Therefore, the protection scope of this application should be subject to the protection scope of the claims.

[0094] This application also provides a flexible DC system. Refer to Figure 8 , Figure 8 A connection schematic diagram of a flexible DC system is provided. The flexible DC system may include a hybrid modular multilevel converter (MMC) connected to each other and the fault handling device as described above.

[0095] As Figure 8 shown, the hybrid MMC may include a circuit breaker, a commutation circuit, a reactor, and a disconnector connected in sequence. Among them, the commutation circuit may be connected to the fault handling device, and the fault handling device executes the above-mentioned fault handling method. Moreover, the circuit breaker and the disconnector may also be connected to the fault handling device, and the fault handling device controls their on / off. In addition, the disconnector is also connected to the power grid at the back end. For more implementation manners of the above components, refer to the foregoing embodiments and will not be elaborated here.

[0096] The flexible DC system provided by this application adopts the fault handling method in the above embodiments, and can solve the technical problems that the fault isolation method in the related flexible DC transmission system has a long time consumption and a complex control process. Compared with the related technology, the beneficial effects of the flexible DC system provided by this application are the same as those of the fault handling method provided by the above embodiments, and other technical features in the flexible DC system are the same as the features disclosed in the fault handling method of the above embodiments, and will not be elaborated here.

[0097] This application also provides a computer-readable storage medium, on which computer-readable program instructions (i.e., computer programs) are stored, and the computer-readable program instructions are used to execute the fault handling method in the above embodiments.

[0098] The computer-readable storage medium provided by this application can be, for example, a USB flash drive, but is not limited to systems or devices of electricity, magnetism, light, electromagnetic, infrared, or semiconductor, or any combination of the above. More specific examples of the computer-readable storage medium can include, but are not limited to: portable computer disks with electrical connections having one or more wires, hard disks, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM) or flash memory, optical fibers, portable compact disk read-only memory (CD-ROM), optical storage devices, magnetic storage devices, etc., or any suitable combination of the above. In this embodiment, the computer-readable storage medium can be any tangible medium that contains or stores a program, which can be used by an instruction execution system or device, or both in combination. The program code contained on the computer-readable storage medium can be transmitted by any suitable medium, including but not limited to: wires, optical cables, RF (Radio Frequency), etc., or any suitable combination of the above.

[0099] The above computer-readable storage medium can be included in the fault handling device; or it can exist separately without being assembled into the fault handling device.

[0100] The above computer-readable storage medium carries one or more programs, and when the above one or more programs are executed by the fault handling device, the fault handling device can implement the above functions defined in the fault handling method disclosed in the embodiments of this application.

[0101] Computer program code for performing the operations of this application can be written in one or more programming languages or combinations thereof. The above programming languages include object-oriented programming languages - such as Java, Smalltalk, C++; and also include conventional procedural programming languages - such as the "C" language or similar programming languages. The program code can be executed entirely on the user's computer, partially on the user's computer, executed as an independent software package, partially on the user's computer and partially on a remote computer, or entirely on a remote computer or server. In the case of a remote computer, the remote computer can be connected to the user's computer through any type of network - including a local area network (LAN) or a wide area network (WAN) - or connected to an external computer, for example, using the Internet connection provided by an Internet service provider.

[0102] The flowcharts and block diagrams in the accompanying drawings illustrate the possible architectures, functions, and operations of methods, apparatuses, systems, and computer program products according to various embodiments of the present application. In this regard, each block in the flowchart or block diagram may represent a module, a segment of a program, or a part of code, which contains one or more executable instructions for implementing the specified logical function. It should also be noted that in some alternative implementations, the functions marked in the blocks may occur in a different order than that marked in the accompanying drawings. For example, two consecutive blocks shown may actually be executed substantially in parallel, and they may sometimes be executed in the reverse order, depending on the functions involved. It should also be noted that each block in the block diagram and / or flowchart, as well as combinations of blocks in the block diagram and / or flowchart, can be implemented by a dedicated hardware-based system that performs the specified functions or operations, or can be implemented by a combination of dedicated hardware and computer instructions.

[0103] The modules described in the embodiments of the present application can be implemented in software or in hardware. Among them, the name of the module does not constitute a limitation to the unit itself in some cases.

[0104] The storage medium provided by the present application is a computer-readable storage medium, on which computer-readable program instructions (i.e., computer programs) for executing the above-mentioned fault handling method are stored, which can solve the technical problems that the fault isolation method in the flexible DC transmission system in the related art is time-consuming and the control process is complex. Compared with the related art, the beneficial effects of the computer-readable storage medium provided by the present application are the same as those of the fault handling method provided by the above embodiments, and will not be elaborated here.

[0105] The above are only some embodiments of the present application, and do not limit the patent scope of the present application accordingly. All equivalent structural transformations made under the technical concept of the present application by using the content of the specification and drawings of the present application, or directly / indirectly applied in other related technical fields, are included in the patent protection scope of the present application.

Claims

1. A fault handling method, characterized in that, Applied to a flexible DC system, the system includes a hybrid modular multilevel converter, and the fault handling method includes: When a DC fault occurs in the hybrid modular multilevel converter, the fault current and fault voltage on the DC side of the hybrid modular multilevel converter are obtained in real time; Judge whether the first preset condition is satisfied according to the fault current and the fault voltage; If the first preset condition is satisfied, the hybrid modular multilevel converter is locked to achieve fault isolation; otherwise, further judge whether the second preset condition is satisfied according to the fault voltage; If the second preset condition is satisfied, negative voltage control is performed on the hybrid modular multilevel converter to achieve fault ride-through; otherwise, the hybrid modular multilevel converter is controlled to operate normally; During the fault ride-through process, judge whether the third preset condition is satisfied according to the fault current; If the third preset condition is satisfied, zero voltage control is performed on the hybrid modular multilevel converter to restore the normal operation of the hybrid modular multilevel converter; otherwise, the hybrid modular multilevel converter is locked to achieve fault isolation.

2. The fault handling method according to claim 1, characterized in that The hybrid modular multilevel converter includes a commutation circuit with a three-phase six-arm structure and a reactor, and the reactor is located on the DC output bus of the commutation circuit; The obtaining the fault current and fault voltage on the DC side of the hybrid modular multilevel converter in real time includes: Obtain the DC output bus voltage of the commutation circuit; Obtain the fault current on the DC side of the hybrid modular multilevel converter according to the first current calculation formula and the DC output bus voltage; where the first current calculation formula is: ， Among them, I dc is the fault current, U dc is the DC output bus voltage, s is the Laplace variable, R eq is the three-phase equivalent resistance of the commutation circuit, R f is the equivalent impedance of the DC fault point, L eq is the three-phase equivalent inductance of the commutation circuit, L dc is the reactance value of the reactor, C eq is the three-phase equivalent capacitance of the commutation circuit; Obtain the fault voltage on the DC side of the hybrid modular multilevel converter according to the fault current and the equivalent impedance of the DC fault point.

3. The fault handling method according to claim 2, wherein The fault current includes DC positive pole current and DC negative pole current, and the fault voltage includes DC positive pole instantaneous voltage and DC negative pole instantaneous voltage; The first preset condition is that the absolute value of the difference between the DC positive pole instantaneous voltage and the DC negative pole instantaneous voltage is less than or equal to the first voltage setting value, and the value of the DC positive pole current or the DC negative pole current is greater than or equal to the first current setting value; The second preset condition is that the absolute value of the sum of the DC positive pole instantaneous voltage and the DC negative pole instantaneous voltage is greater than or equal to the second voltage setting value.

4. The fault handling method according to claim 1, wherein The hybrid modular multilevel converter includes a commutation circuit with a three-phase six-arm structure and a reactor, the reactor is located on the DC output bus of the commutation circuit, each arm in the commutation circuit includes a plurality of sub-modules, the plurality of sub-modules include at least one half-bridge sub-module and at least one full-bridge sub-module, and the proportion of the full-bridge sub-modules in the plurality of sub-modules is greater than or equal to 50%; The judging whether the third preset condition is satisfied according to the fault current during the fault ride-through process includes: During the fault ride-through process, obtain the DC output bus voltage of the commutation circuit; Obtain the fault current on the DC side of the hybrid modular multilevel converter according to the second current calculation formula and the DC output bus voltage; wherein, the second current calculation formula is: ， Among them, I dc is the fault current, h is the full-bridge sub-module ratio of each arm in the commutation circuit, h≥ 50%, U dc is the DC output bus voltage, s is the Laplace variable, L eq is the three-phase equivalent inductance of the commutation circuit, L dc is the reactance value of the reactor; Judge whether the third preset condition is satisfied according to the fault current.

5. The fault handling method according to claim 4, characterized in that The fault current includes the DC positive current and the DC negative current; The third preset condition includes that the value of the DC positive current is less than or equal to the second current setting value, and the value of the DC negative current is less than or equal to the second current setting value, and the second current setting value is determined based on the rated current of the hybrid modular multilevel converter.

6. The fault handling method according to claim 2 or 4, characterized in that, The structures of all arms in the commutation circuit are the same. Each arm includes N sub-modules, and the N sub-modules include at least one half-bridge sub-module and at least one full-bridge sub-module. Among them, the capacitance value of the single capacitor in the half-bridge sub-module is C 0h , and the capacitance value of the single capacitor in the full-bridge sub-module is C 0f ; The method further includes: According to the equivalent resistance calculation formula and the arm resistances of each arm R arm the three-phase equivalent resistance is obtained R eq , and the equivalent resistance calculation formula is as follows: ， According to the equivalent inductance calculation formula and the leg inductances of each leg L arm The three-phase equivalent inductance is obtained L eq , and the equivalent inductance calculation formula is as follows: ， The three-phase equivalent capacitance is obtained based on the equivalent capacitance calculation formula and the number N of sub-modules in each arm C eq , and the equivalent capacitance calculation formula is as follows: ， Among them, ε is the equivalent capacitance coefficient, which is set corresponding to different stages of the DC fault, C 0 =C 0h +C 0f .

7. The fault handling method according to claim 1, wherein The zero-voltage control of the hybrid modular multilevel converter to restore the normal operation of the hybrid modular multilevel converter includes: Perform zero-voltage control on the hybrid modular multilevel converter and judge whether the DC fault is cleared; If it has been cleared, determine that the DC fault is a transient fault and control the hybrid modular multilevel converter to operate normally; If it has not been cleared, determine that the DC fault is a permanent fault, and disconnect the circuit breaker on the AC side and the disconnector on the DC side of the hybrid modular multilevel converter to achieve fault isolation.

8. A fault handling device, characterized in that, The device includes a memory, a processor, and a computer program stored on the memory and running on the processor, and the computer program is configured to implement the steps of the fault handling method according to any one of claims 1 to 7.

9. A flexible DC system, characterized in that, The flexible DC system includes a hybrid modular multilevel converter modular multilevel converter connected to each other and the fault handling device according to claim 8.

10. A storage medium, characterized in that The storage medium is a computer-readable storage medium, and a computer program is stored on the storage medium, and when the computer program is executed by the processor, the steps of the fault handling method according to any one of claims 1 to 7 are implemented.

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