Fault handling method, equipment, flexible DC system and storage medium

Abstract

The present application discloses a fault handling method, device, flexible DC system and storage medium, which relate to the field of system control technology. Applied to a flexible DC system including a hybrid MMC, the method includes: when an MMC DC fault occurs, obtaining the DC side fault current and fault voltage in real time; judging whether a first preset condition is met based on the fault current and fault voltage; if so, performing blocking control on the MMC to achieve fault isolation; otherwise, continuing to judge whether a second preset condition is met based on the fault voltage; if so, performing negative voltage control on the MMC to achieve fault ride-through; otherwise, controlling the MMC to operate normally; during the fault ride-through process, judging whether a third preset condition is met based on the fault current; if so, performing zero voltage control on the MMC to restore the normal operation of the MMC; otherwise, blocking control on the MMC to achieve fault isolation. The present application provides a fast and reliable flexible DC fault handling method at the system level.

Description

Technical Field

[0001] The present application relates to the field of system control technology, and in particular to a fault handling method, device, flexible direct current system, and storage medium. Background Art

[0002] Modular Multilevel Converters (MMCs) are widely used in multi-terminal flexible DC transmission systems. Currently, fault isolation solutions for flexible DC grids include blocking all converter stations to clear the fault or using non-blocking fault ride-through. However, these solutions significantly impact fault clearing time, resulting in a long process, and the fault ride-through control process is complex. Summary of the Invention

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

[0004] To achieve the above objectives, the present application proposes a fault handling method, which is applied to a flexible DC system. The system includes a hybrid modular multilevel converter (MMC). The fault handling method includes:

[0005] When a DC fault is detected in the hybrid MMC, the fault current and fault voltage on the DC side of the hybrid MMC are obtained in real time;

[0006] Determining whether a first preset condition is met according to the fault current and the fault voltage;

[0007] If the first preset condition is met, the hybrid MMC is locked to achieve fault isolation; otherwise, it is further determined whether the second preset condition is met based on the fault voltage;

[0008] If the second preset condition is met, negative voltage control is performed on the hybrid MMC to achieve fault ride-through; otherwise, the hybrid MMC is controlled to operate normally;

[0009] During the fault ride-through process, determining whether a third preset condition is met based on the fault current;

[0010] If the third preset condition is met, the hybrid MMC is subjected to zero voltage control to restore the normal operation of the hybrid MMC; otherwise, the hybrid MMC is subjected to blocking control to achieve fault isolation.

[0011] In one embodiment, the hybrid MMC includes a commutation circuit with a three-phase six-bridge-arm structure and a reactor, wherein the reactor is located on a DC output bus of the commutation circuit;

[0012] Real-time acquisition of fault current and fault voltage on the DC side of the hybrid MMC, including:

[0013] Obtaining the DC output bus voltage of the commutation circuit;

[0014] The fault current on the DC side of the hybrid MMC is obtained according to the first current calculation formula and the DC output bus voltage; wherein the first current calculation formula is:

[0015] ,

[0016] in, 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;

[0017] The fault voltage on the DC side of the hybrid MMC is obtained according to the fault current and the equivalent impedance of the DC fault point.

[0018] In one embodiment, the fault current includes a DC positive current and a DC negative current, and the fault voltage includes a DC positive instantaneous voltage and a DC negative instantaneous voltage;

[0019] 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;

[0020] 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.

[0021] In one embodiment, a hybrid MMC includes a commutation circuit with a three-phase six-bridge-arm structure and a reactor. The reactor is located on a DC output busbar of the commutation circuit. Each bridge arm in the commutation circuit includes multiple submodules. The multiple submodules include at least one half-bridge submodule and at least one full-bridge submodule. The full-bridge submodule accounts for greater than or equal to 50% of the multiple submodules.

[0022] During the fault ride-through process, whether the third preset condition is met is determined based on the fault current, including:

[0023] During the fault ride-through process, the DC output bus voltage of the commutation circuit is obtained;

[0024] The fault current on the DC side of the hybrid MMC is obtained according to the second current calculation formula and the DC output bus voltage; wherein the second current calculation formula is:

[0025] ,

[0026] in, I dc is the fault current, h is the proportion of the full bridge submodule of each bridge 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;

[0027] It is determined whether the third preset condition is met according to the fault current.

[0028] In one embodiment, the fault current includes a DC positive current and a DC negative current;

[0029] 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 MMC.

[0030] In one embodiment, the structure of each bridge arm in the commutation circuit is consistent, and each bridge arm includes N submodules, and the N submodules include at least one half-bridge submodule and at least one full-bridge submodule, wherein the capacitance of a single capacitor in the half-bridge submodule is C 0h , the capacitance of a single capacitor in the full-bridge submodule is C 0f ;

[0031] The method also includes:

[0032] According to the equivalent resistance calculation formula and the bridge arm resistance of each bridge arm R arm Get the three-phase equivalent resistance R eq , the equivalent resistance calculation formula is:

[0033] ,

[0034] According to the equivalent inductance calculation formula and the bridge arm inductance of each bridge armL arm Get the three-phase equivalent inductance L eq , the equivalent inductance calculation formula is:

[0035] ,

[0036] According to the equivalent capacitance calculation formula and the number of sub-modules N in each bridge arm, the three-phase equivalent capacitance is obtained. C eq , the equivalent capacitance calculation formula is:

[0037] ,

[0038] in, ε is the equivalent capacitance coefficient, which is set based on the different stages of DC fault. C 0 =C 0h +C 0f .

[0039] In one embodiment, performing zero voltage control on a hybrid MMC to restore normal operation of the hybrid MMC includes:

[0040] Perform zero voltage control on the hybrid MMC and determine whether the DC fault has been cleared;

[0041] If it has been cleared, the DC fault is determined to be a transient fault, and the hybrid MMC is controlled to operate normally;

[0042] If it is not cleared, the DC fault is determined to be 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.

[0043] To achieve the above objectives, the present application also proposes a fault handling device, comprising a memory, a processor, and a computer program stored in the memory and running on the processor, wherein the computer program is configured to implement the steps of the above-mentioned fault handling method.

[0044] In addition, to achieve the above objectives, the present application also proposes a flexible DC system, including interconnected hybrid modular multilevel converters MMC and the above-mentioned fault handling device.

[0045] In addition, to achieve the above-mentioned purpose, the present application also proposes a storage medium, which is a computer-readable storage medium. A computer program is stored on the storage medium, and when the computer program is executed by a processor, the steps of the fault handling method as described above are implemented.

[0046] One or more technical solutions proposed in this application have at least the following technical effects:

[0047] A fault handling method is proposed. When a DC fault is detected in a hybrid MMC in a flexible DC system, the method determines whether a first preset condition is met based on the real-time DC side fault current and fault voltage. If the first preset condition is met, the hybrid MMC is blocked to isolate the fault. This allows for rapid and reliable fault removal, ensuring grid safety and improving system reliability. If the first preset condition is not met, the method determines whether a second preset condition is met based on the real-time fault voltage. If the second preset condition is met, the hybrid MMC is negatively voltage controlled to achieve fault ride-through. During the fault ride-through process, the real-time fault current determines whether a third preset condition is met. If the third preset condition is met, the hybrid MMC is zero-voltage controlled to restore normal operation. This allows for rapid power restoration for transient faults, minimizing system outage time and maximizing grid voltage stability and power transmission. If the third preset condition is not met, the hybrid MMC is blocked to isolate the fault. This allows for fault removal when the fault ride-through strategy fails, ensuring grid safety and further improving system reliability. The method of the present application adopts a fault handling logic strategy that combines multi-level judgment and control, performs system operations corresponding to different fault conditions, and provides a fast and reliable flexible DC fault handling method at the system level. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The accompanying drawings herein are incorporated into and constitute a part of the specification, illustrate embodiments consistent with the present application, and together with the specification, are used to explain the principles of the present application. In order to more clearly illustrate the technical solutions in the embodiments of the present application or related technologies, the following is a brief introduction to the drawings required for use in the embodiments or related technology descriptions. Obviously, for those of ordinary skill in the art, other drawings can be obtained based on these drawings without inventive work.

[0049] Figure 1 A flowchart of the first embodiment of the fault handling method provided by this application;

[0050] Figure 2 A flowchart of the second embodiment of the fault handling method provided by this application;

[0051] Figure 3 This is the equivalent circuit model of the hybrid MMC when a fault occurs in the second embodiment of the fault handling method provided by this application;

[0052] Figure 4 The equivalent circuit model of the hybrid MMC during the fault ride-through process in the second embodiment of the fault handling method provided by this application;

[0053] Figure 5 Based on Figure 4 The superposition network equivalent model is obtained by performing equivalent transformation on the equivalent circuit model of ;

[0054] Figure 6 A flowchart of an application example of the fault handling method provided in an embodiment of the present application;

[0055] Figure 7 A schematic diagram of the hardware structure of a fault handling device involved in an embodiment of the present application;

[0056] Figure 8 This is a connection diagram of the flexible DC system involved in the embodiment of the present application.

[0057] The realization of the objectives, functional features and advantages of this application will be further explained in conjunction with embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0058] It should be understood that the specific embodiments described herein are merely for explaining the technical solutions of the present application and are not intended to limit the present application. In order to better understand the technical solutions of the present application, the following detailed description will be given in conjunction with the accompanying drawings and specific implementation methods.

[0059] HVDC Flexible is a high-voltage direct current (HVDC) transmission technology based on power electronics. In multi-terminal HVDC Flexible systems, modular multilevel converters (MMCs) are widely used in a variety of applications, including long-distance, high-capacity power transmission, asynchronous grid interconnection, and offshore wind power transmission, due to their strong controllability, lack of reactive power compensation requirements, and ability to power passive networks. Compared to AC systems, DC systems exhibit a faster rise rate, higher peak values, and a lack of natural zero-crossing points, making fault isolation more difficult. This has hindered the development and application of HVDC Flexible technology.

[0060] Currently, there are two main fault isolation solutions for flexible DC grids: one is a half-bridge MMC and DC circuit breaker, which can selectively disconnect faulty lines, but is difficult to develop and expensive. The other is an MMC with self-fault clearing capabilities and a fast disconnector. This solution can block fault currents, but requires locking all converter stations in the DC grid, which leads to power transmission interruption and requires a long time for the converter stations to restart after the fault is cleared. Although related technologies use a non-lockout fault ride-through method, specifically using a submodule with negative level output capability to reduce the DC voltage output of the converter station, thereby limiting the short-circuit current to zero to ensure voltage stability and power transmission, this method also has the problem that the fault clearing time is significantly affected by the controlled DC voltage, resulting in a long time, and the fault ride-through control process is relatively complex.

[0061] It can be seen that the fault isolation method of the flexible direct current transmission system in the related art has the problems of being time-consuming, having a complex control process and being costly.

[0062] To address the above issues, embodiments of the present application provide a fault handling method, device, flexible DC system, and storage medium. For ease of description, the following detailed description will be based on the fault handling device in the flexible DC system.

[0063] It should be noted that the fault handling device is a computing service device with data processing, network communication, and program execution functions, such as a server, network terminal, embedded computer, industrial personal 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 application, it can also implement other control operations such as on-off control of switching devices, output regulation of converters, and system start-stop control, etc., which are not specifically limited here.

[0064] An embodiment of the present application provides a fault handling method.

[0065] In the first embodiment of the fault handling method of the present application, refer to Figure 1 , Figure 1 This is a flowchart of the first embodiment of the fault handling method of this application. The 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. The fault handling method can include steps S10 to S80:

[0066] Step S10: When a DC fault is detected in the hybrid MMC, a fault current and a fault voltage on the DC side of the hybrid MMC are acquired in real time.

[0067] It should be noted that a hybrid MMC refers to a modular multilevel converter in which each bridge arm contains two or more different sub-modules with different structures, such as a combination of a half-bridge sub-module (HBSM) and a full-bridge sub-module (FBSM). A DC fault refers to a short-circuit fault occurring on the DC side of a hybrid MMC, such as an inter-pole short circuit or single-pole grounding. During a fault, the fault current rapidly rises, potentially damaging system equipment. The fault current refers to the current flowing through the fault point when a DC fault occurs in a hybrid MMC, and the fault voltage refers to the instantaneous change in the DC side voltage when a DC fault occurs in a hybrid MMC.

[0068] It should also be noted that the fault current may include the positive and negative currents on the DC side of the hybrid MMC. Real-time acquisition of the fault current can be calculated by real-time detection of the DC bus voltage of the hybrid MMC. The fault current may also include the current at the fault point, which can be detected by a separately provided current detection module or circuit. The fault voltage may include the instantaneous positive and negative voltages on the DC side of the hybrid MMC. Real-time acquisition of the fault voltage can be further calculated based on the calculated fault current. The fault voltage may 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.

[0069] Step S20: determining whether a first preset condition is satisfied according to the fault current and the fault voltage.

[0070] It should be noted that the first preset condition is set based on the fault current and fault voltage. It can be a threshold range requirement directly targeting both the fault current and the fault voltage. Accordingly, satisfying the first preset condition can be a direct requirement that the fault current and the fault voltage simultaneously satisfy their respective preset threshold ranges. That is, the first preset condition is satisfied only 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. Alternatively, it can be a threshold range requirement indirectly related to the fault current and the fault voltage. Accordingly, satisfying the first preset condition can be a requirement that an indirect value (such as the mean, sum, difference, or absolute value) derived from the fault current and an indirect value (such as the mean, sum, difference, or absolute value) derived from the fault voltage simultaneously satisfy their respective preset threshold ranges. That is, the first preset condition is satisfied only when the indirect value derived from the fault current and the indirect value derived from the fault voltage also satisfy the threshold range in the first preset condition. The preset threshold range can be set based on actual needs. For example, a current threshold range or a voltage threshold range can be set based on the voltage and current displayed by the hybrid MMC during a serious fault. If the current or voltage threshold range is not within the range, it is not a serious fault, or if either value is not within the corresponding preset threshold range, it is not a serious fault. Alternatively, the corresponding threshold range can be calculated and set based on the voltage and current displayed by the hybrid MMC during a serious fault. In actual applications, the first preset condition can be set based on actual needs and is not specifically limited here.

[0071] Step S30: If the first preset condition is met, the hybrid MMC is locked to achieve fault isolation.

[0072] It should be noted that fault isolation refers to the rapid identification and isolation of the faulty area after a fault is detected, preventing the fault from spreading to the entire system. Lockout control shuts down the power devices of all submodules in a hybrid MMC, forcing the bypass of the submodule's capacitors, thereby stopping the hybrid MMC and isolating the fault current.

[0073] If the current hybrid MMC satisfies the first preset condition based on the fault current and fault voltage, it means that the DC fault of the hybrid MMC is a serious fault, such as an inter-pole short circuit fault. If fault ride-through is performed, it may cause more equipment failures. Therefore, the control strategy of locking the converter can be used to lock the hybrid MMC to remove the fault and achieve fault isolation.

[0074] Step S40: If the first preset condition is not met, further determine whether the second preset condition is met based on the fault voltage.

[0075] It should be noted that, if the current hybrid MMC is determined to not meet the first preset condition based on the fault current and fault voltage, it means that the DC fault of the hybrid MMC is not a serious fault. At this time, subsequent operations can be continued, and specifically, whether the second preset condition is met is further determined based on the fault voltage obtained in real time.

[0076] It should also be noted that the second preset condition is set separately based on the fault voltage. It can be a threshold range requirement directly related to the fault voltage, where satisfying the second preset condition requires the fault voltage to meet the corresponding preset threshold range. Alternatively, it can be a threshold range requirement indirectly related to the fault voltage, where satisfying the second preset condition requires an indirect value derived from the fault voltage (such as the mean, sum, difference, or absolute value) to meet the corresponding preset threshold range. The preset threshold range can be set based on actual needs. For example, the threshold range can be set based on the voltage exhibited by the hybrid MMC during one or more general faults or transient faults, where fault ride-through can be performed. If the voltage falls within this range, the DC fault, while not a critical fault, is nonetheless eligible for fault ride-through. If the voltage falls outside this range, the DC fault is neither a critical fault nor one requiring fault ride-through, but rather a minor fluctuation during normal operation, which will not impact the system or internal devices if normal operation continues. Alternatively, the threshold range can be calculated and set based on the voltage exhibited by the hybrid MMC during general faults or transient faults, where fault ride-through can be performed. In practical applications, the second preset condition can be set according to actual needs and is not specifically limited here.

[0077] Step S50: If the second preset condition is met, negative voltage control is performed on the hybrid MMC to achieve fault ride-through.

[0078] It's important to note that fault ride-through (FRT) refers to a system's ability to maintain operation and continue power transmission through control strategy adjustments during a fault, preventing system crashes or large-scale power outages caused by the fault. Compared to fault isolation, fault ride-through focuses on operating with the fault, while fault isolation focuses on rapidly clearing the fault. Negative voltage control refers to a DC voltage control mode that actively reduces the DC side voltage by controlling the hybrid MMC's output voltage to a negative level (e.g., discharging the capacitors of each submodule) and dropping the voltage below zero to match the voltage at the fault point or suppress overcurrent.

[0079] If the current hybrid MMC satisfies the second preset condition based on the fault voltage, it means that the DC fault of the hybrid MMC is not a serious fault, but a general fault or transient fault, which can be subjected to fault ride-through, such as a single-pole grounding fault. If the converter is directly blocked, power transmission will be interrupted, and it will take a long time to restart the converter. Therefore, a negative DC voltage control strategy can be used to control the negative voltage of the hybrid MMC, perform non-blocking voltage reduction and current limiting, and achieve fault ride-through.

[0080] Step S60: If the second preset condition is not met, the hybrid MMC is controlled to operate normally.

[0081] It should be noted that, if the current hybrid MMC does not meet the second preset condition based on the fault voltage, it means that the DC fault of the hybrid MMC is not a serious fault, nor is it a fault that requires fault ride-through. It may be considered that the DC fault is just a small fluctuation. At this time, normal control of the hybrid MMC can be maintained to enable the hybrid MMC to continue normal operation.

[0082] Step S70: During the fault ride-through process, determine whether a third preset condition is met based on the fault current.

[0083] It should be noted that fault ride-through has a current-limiting effect. During this period, step S10 will continue to acquire the fault current in real time. The third preset condition is set separately based on the fault current. This can be a threshold range requirement directly related to the fault current, where satisfying the third preset condition requires the fault current to fall within the corresponding preset threshold range. Alternatively, it can be a threshold range requirement indirectly related to the fault current, where satisfying the third preset condition requires an indirect value derived from the fault current (such as the mean, sum, difference, or absolute value) to fall within the corresponding preset threshold range. The preset threshold range can be set based on actual needs. For example, the threshold range can be set based on the current displayed by the hybrid MMC during normal operation or with minor fluctuations. If it is within this range, it indicates that the current DC fault is not a serious fault, nor is it a fault that can be subjected to fault ride-through. The current fault current has returned to a 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 indicates that although the current DC fault can temporarily maintain power transmission through fault ride-through, fault ride-through is not a long-term solution because the fault current does not effectively decrease or return to the normal range during the fault ride-through period. This indicates that the current DC fault is not a serious fault but a permanent fault, or that the aforementioned fault ride-through current limiting has failed, and further fault response measures should be taken subsequently. Alternatively, the corresponding threshold range can be calculated and set based on the current displayed by the hybrid MMC during normal operation or with minor fluctuations. In actual applications, the second preset condition can be set based on actual needs and is not specifically limited here.

[0084] Step S80: If the third preset condition is met, performing zero voltage control on the hybrid MMC to restore the normal operation of the hybrid MMC;

[0085] If the third preset condition is not met, the hybrid MMC is locked to achieve fault isolation.

[0086] 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 so that the capacitors of the sub-modules are neither charged nor discharged, thereby achieving a DC voltage control mode in which the output voltage is zero. Figure 1 As shown, if the judgment result of step S70 is no, the process can directly jump to step S30 to perform the step of locking control on the hybrid MMC to implement fault isolation.

[0087] If the current hybrid MMC satisfies the third preset condition based on the fault current, it means that the DC fault of the hybrid MMC has been successfully restored to a normal state through the current limiting effect of the fault ride-through. In order to shorten the fault recovery time, once the third preset condition is found to be met during the fault ride-through, the control strategy of the hybrid MMC can be immediately switched to perform zero voltage control on the hybrid MMC using a zero DC voltage control strategy to avoid reverse current shock and restore the normal operation of the hybrid MMC. After the hybrid MMC establishes DC voltage, the line active power transmission is restored, thereby restoring the normal operation of the entire system.

[0088] If the fault current determines that the hybrid MMC does not meet the third preset condition, this indicates that the hybrid MMC's DC fault has not been restored to normal by the current limiting function of the fault ride-through function. This may be because the DC fault is permanent or the current limiting function of the fault ride-through function has failed. In this case, the hybrid MMC can be blocked using a blocking converter control strategy to completely eliminate the fault and achieve fault isolation. After the fault is resolved, if power is restored, the hybrid MMC can be restarted.

[0089] It can be understood that the method of this embodiment takes into account the various types of overhead line faults in the flexible DC system, the significant environmental impact, and the fact that most of them are single-pole grounding faults. By using a non-lockout fault ride-through strategy, when it is determined that the first preset condition (not a serious fault) is not met but the second preset condition is met, fault ride-through is performed, which can maximize the voltage stability and power transmission of the back-end DC grid and quickly restore power supply. In this method, a locking control strategy is also used to isolate the fault for serious faults or when the non-lockout fault ride-through strategy fails (a permanent fault), which can reliably eliminate the fault and ensure the safety of the back-end DC grid.

[0090] This embodiment provides a fault handling method. When a DC fault is detected in a hybrid MMC in a flexible DC system, the method determines whether a first preset condition is met based on the real-time acquired DC side fault current and fault voltage. If the first preset condition is met, the hybrid MMC is locked to isolate the fault. This allows for rapid and reliable fault removal, ensuring grid safety and improving system reliability. If the first preset condition is not met, the method determines whether a second preset condition is met based on the real-time acquired fault voltage. If the second preset condition is met, the hybrid MMC is negatively voltage controlled to achieve fault ride-through. During the fault ride-through process, the method determines whether a third preset condition is met based on the real-time acquired fault current. If the third preset condition is met, the hybrid MMC is zero-voltage controlled to restore normal operation of the hybrid MMC. This allows for rapid power restoration for transient faults, minimizes system outage duration, and maximizes grid voltage stability and power transmission. If the third preset condition is not met, the hybrid MMC is locked to isolate the fault. This allows for fault removal when the fault ride-through strategy fails, ensuring grid safety and further improving system reliability. The method of the present application adopts a fault handling logic strategy that combines multi-level judgment and control, performs system operations corresponding to different fault conditions, and provides a fast and reliable flexible DC fault handling method at the system level.

[0091] In one possible implementation, Figure 1 As shown, the step S30 of "locking and controlling the hybrid MMC to achieve fault isolation" may include step S31:

[0092] In step S31 , the hybrid MMC is locked and the circuit breaker on the AC side and the disconnector on the DC side of the hybrid MMC are disconnected to achieve fault isolation.

[0093] It should be noted that in a hybrid MMC of a flexible DC system, in addition to the commutation circuit, which plays a key and primary role, a circuit breaker can be installed on the AC side to isolate the AC power source from the commutation circuit, and an isolating switch can be installed on the DC side to isolate the commutation circuit from the power grid. To ensure that the hybrid MMC completely isolates the fault and maintains the overall reliability and safety of the system, the switching devices in each bridge arm of the commutation circuit can be locked while disconnecting the AC circuit breaker and the DC isolating switch to prevent the fault from damaging other equipment on the front and back ends of the hybrid MMC.

[0094] In this embodiment, while the hybrid MMC is locked, the circuit breaker on the AC side of the hybrid MMC and the isolating switch on the DC side can be disconnected to prevent the DC fault from damaging the hybrid MMC or its front-end and back-end equipment, avoid the impact of the fault from expanding, and prevent adverse effects on system operation and recovery after the fault is resolved.

[0095] 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, the same or similar contents as those in the above embodiment can be referred to the above introduction and will not be repeated hereafter. Figure 2 , Figure 2 This is a flowchart of the second embodiment of the fault handling method of this application. Step S80 may include steps S81 to S84:

[0096] Step S81, performing zero voltage control on the hybrid MMC;

[0097] Step S82, determining whether the DC fault is cleared;

[0098] Step S83: If the fault is cleared, the DC fault is determined to be a transient fault, and the hybrid MMC is controlled to operate normally.

[0099] Step S84: If the fault is not cleared, the DC fault is determined to be a permanent fault, and the circuit breaker on the AC side and the disconnector on the DC side of the hybrid MMC are disconnected to achieve fault isolation.

[0100] It should be noted that during the fault ride-through process, if the third preset condition is met based on the fault current, the DC voltage control mode of the hybrid MMC can be switched, specifically switching the negative voltage control to zero voltage control. At the same time, a system judgment process can be added to specifically determine whether the DC fault has been cleared. Specifically, the judgment can be performed based on the output results of a fault detection device within the system. If the DC fault has been cleared, it indicates that the DC fault is a transient fault. At this time, the hybrid MMC can be controlled normally to restore normal operation of the system. If the DC fault has not been cleared, it indicates 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 methods, please refer to the description of the aforementioned embodiment and will not be repeated here.

[0101] As you can understand, by adding a fault-clearing judgment process, it's clear whether the DC fault is transient or permanent, facilitating further system recovery or fault isolation. This not only makes the fault handling process more intuitive and clear, but also further ensures system reliability and safety.

[0102] In one feasible embodiment, the hybrid MMC may include a three-phase six-bridge-arm commutation circuit and a reactor, with the reactor located on the DC output bus of the commutation circuit. The reactor may be located on the positive or negative DC bus of the DC output, without specific limitation herein.

[0103] Correspondingly, the step S10 of "real-time acquisition of the fault current and fault voltage on the DC side of the hybrid MMC" may include steps S11 to S13:

[0104] Step S11, obtaining the DC output bus voltage of the commutation circuit U dc ;

[0105] Step S12: According to the first current calculation formula and the DC output bus voltage U dc The fault current on the DC side of the hybrid MMC is obtained; wherein the first current calculation formula is:

[0106] ,

[0107] in, 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;

[0108] Step S13: Based on the fault current and the equivalent impedance of the DC fault point R f Obtain the fault voltage on the DC side of the hybrid MMC.

[0109] It should be noted that the discharge path formed by the hybrid MMC after a DC fault occurs is the main channel for energy release in the flexible DC system, which directly affects the magnitude of the DC short-circuit current. Since the hybrid MMC needs to go through two judgment processes before deciding whether to perform fault ride-through during the fault handling process, that is, the DC control strategy of the hybrid MMC needs to switch to the fault ride-through mode after a period of time, so when a DC fault occurs in the hybrid MMC, including a serious fault such as an inter-pole short circuit fault or a general fault such as a single-pole grounding fault, the hybrid MMC can be equivalent to an RLC discharge circuit with initial capacitor energy storage, such as Figure 3 FIG. 1 shows an equivalent circuit model of the hybrid MMC when a fault occurs in this embodiment.

[0110] like Figure 3 As shown in FIG, after detecting a DC fault in the hybrid MMC and before performing fault ride-through, the equivalent model of the hybrid MMC includes three-phase equivalent inductors connected in sequence. L eq , three-phase equivalent resistance R eq , three-phase equivalent capacitance C eq , Reactor L dc , equivalent impedance of DC fault point R f In this case, the real-time fault current can be obtained based on the DC output bus voltage of the commutation circuit. U dc The first current calculation formula is used to calculate the fault voltage, so as to perform the judgment of step S20 or S40.

[0111] In an optional embodiment, the structure of each bridge arm in the commutation circuit is consistent, each bridge arm includes N submodules, and the N submodules include at least one half-bridge submodule and at least one full-bridge submodule, wherein the capacitance of a single capacitor in the half-bridge submodule is C 0h , the capacitance of a single capacitor in the full-bridge submodule is C 0f .

[0112] Correspondingly, the fault handling method may further include steps S01 to S03:

[0113] Step S01: According to the equivalent resistance calculation formula and the bridge arm resistance of each bridge arm R arm Get the three-phase equivalent resistance R eq , the equivalent resistance calculation formula is:

[0114] ,

[0115] Step S02: According to the equivalent inductance calculation formula and the bridge arm inductance of each bridge arm L arm Get the three-phase equivalent inductance L eq , the equivalent inductance calculation formula is:

[0116] ,

[0117] Step S03: Obtain the three-phase equivalent capacitance according to the equivalent capacitance calculation formula and the number N of submodules in each bridge arm. C eq , the equivalent capacitance calculation formula is:

[0118] ,

[0119] in, ε is the equivalent capacitance coefficient, which is set based on the different stages of DC fault. C 0 =C 0h +C 0f .

[0120] It should be noted that the above steps S01 to S03 can be executed before step S10, or can be performed along with the fault handling process. Each time the fault current and fault voltage need to be obtained in real time before the judgment of the preset condition is executed, the three-phase equivalent resistance of the commutation circuit is determined. R eq , three-phase equivalent inductance L eq and three-phase equivalent capacitance C eq Afterwards, the fault current is obtained by quick calculation.

[0121] Furthermore, the fault current may include a DC positive current I dP and DC negative current I dN , the fault voltage may include the DC positive transient voltage U dP and DC negative instantaneous voltage U dN .

[0122] In a specific embodiment, the first preset condition can be the DC positive instantaneous voltage U dP and DC negative instantaneous voltage U dN The absolute value of the difference is less than or equal to the first voltage setting value Ud.set1 , and the DC positive current U dP or DC negative current U dN The value is greater than or equal to the first current setting value I d.set1 .

[0123] It should be noted that, based on the detailed description above, satisfying the first preset condition indicates a serious fault in the hybrid MMC. Therefore, the first preset condition can be set based on the specific serious fault scenario. For example, the first preset condition is set based on an inter-pole short-circuit fault to ensure reliable operation of the hybrid MMC's locking control when an inter-pole short-circuit occurs. An inter-pole short-circuit fault in this context refers to abnormal conduction between the two poles (positive and negative) on the DC side, causing current to flow directly from one pole to the other rather than through the normal load path. This fault may be caused by insulation failure, equipment breakdown, external environmental damage (such as lightning strikes or foreign object intrusion), or system abnormalities.

[0124] Among them, the first preset condition can be , where && represents logical AND and || represents logical OR. Step S20 may include determining whether the current If so, execute step S30; otherwise, execute step S40.

[0125] In another specific embodiment, the second preset condition can be the DC positive instantaneous voltage U dP and DC negative instantaneous voltage U dN The absolute value of the sum is greater than or equal to the second voltage setting value U d.set2 .

[0126] It should be noted that, based on the foregoing specific description, fault ride-through needs to be performed when the second preset condition is met. Therefore, the second preset condition can be set accordingly based on the general fault or transient fault situation in which fault ride-through can be performed. For example, the second preset condition is set based on a single-pole grounding fault to ensure that the fault ride-through of the hybrid MMC can be reliably operated when a single-pole grounding fault occurs. The single-pole grounding fault here refers to abnormal conduction between a certain pole (positive or negative pole) on the DC side and the ground (earth), resulting in current leakage through the grounding path. This type of fault is usually caused by factors such as equipment insulation defects, external force damage (such as accidental damage during construction), cable aging or lightning strikes, and is one of the more common and more harmful fault types in flexible DC systems.

[0127] Specifically, the second preset condition is Step S40 may include determining whether the current If so, execute step S50; otherwise, execute step S60.

[0128] The fault handling method provided in this embodiment is based on the electrical quantity characteristics obtained in real time after the fault, such as the instantaneous value of voltage and the positive and negative currents. After setting the first preset condition and the second preset condition accordingly, different control strategies are dynamically matched to achieve rapid blocking and fault isolation of severe faults, as well as fault ride-through and rapid recovery of transient faults. This can ensure the safe and stable operation of the back-end power grid, minimize power outage time, and improve the system power supply reliability and power supply duration.

[0129] In another feasible embodiment, the hybrid MMC may include a commutation circuit with a three-phase six-bridge-arm structure and an inductor, wherein the inductor is located on the DC output bus of the commutation circuit. Each bridge arm in the commutation circuit includes multiple sub-modules, and the multiple sub-modules include at least one half-bridge sub-module and at least one full-bridge sub-module, wherein the full-bridge sub-module accounts for greater than or equal to 50% of the multiple sub-modules.

[0130] The structures of the bridge arms can be the same, that is, the total number of submodules in each bridge arm can be the same, and the proportion of half-bridge submodules and full-bridge submodules can also be the same. N HB Half-bridge submodule (HBSM), N FB The number of sub-modules in each bridge arm is N=N HB +N FB .

[0131] Correspondingly, step S70 may include steps S71 to S73:

[0132] Step S71, during the fault ride-through process, obtaining the DC output bus voltage of the commutation circuit;

[0133] 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; wherein the second current calculation formula is:

[0134] ,

[0135] in, I dc is the fault current, h is the proportion of the full bridge submodule of each bridge arm in the commutation circuit, h≥ 50%, U dc is the DC output bus voltage, s is the Laplace variable, Leq is the three-phase equivalent inductance of the commutation circuit, L dc is the reactance value of the reactor;

[0136] Step S73: Determine whether a third preset condition is met based on the fault current.

[0137] It should be noted that during the DC fault ride-through of the hybrid MMC, the full-bridge submodules in each bridge 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 bridge arm is set to the lower limit of the voltage adjustable range. To output a negative DC voltage, the full-bridge submodule ratio should be greater than 50%, so this fault handling method is more suitable for full-bridge submodule ratios. h≥ 50% hybrid MMC. Based on the above example, the full bridge submodule accounts for h for:

[0138] .

[0139] Based on this, during the DC fault ride-through period, the hybrid MMC can suppress the DC short-circuit current by using the sub-module to actively output a negative level, which essentially changes the equivalent circuit model of the hybrid MMC when a fault occurs, such as Figure 4 FIG. 4 shows an equivalent circuit model of the hybrid MMC during the fault ride-through process in this embodiment.

[0140] like Figure 4 As shown 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 resistance R eq , three-phase equivalent capacitance C eq , equivalent negative voltage source , Reactor L dc , equivalent impedance of DC fault point R f According to the superposition theorem, the circuit structure change caused by the DC fault ride-through period can be equivalent to the superposition of an initial network before the DC fault ride-through and an additional network during the DC fault ride-through period, such as Figure 5 Shown is based on Figure 4 The superposition network equivalent model is obtained by performing equivalent transformation on the equivalent circuit model.

[0141] like Figure 5As shown, the left side is the initial network, and the right side is the additional network. The "+" between the two indicates network superposition. Due to the high-frequency characteristics of impedance, the effects of resistance and capacitance can be ignored during the DC fault ride-through process. The fault current expression when the branch is current-limited and voltage-reduced during the fault ride-through process is obtained, that is, the second current calculation formula:

[0142] ;

[0143] In this case, the real-time fault current can be obtained based on the DC output bus voltage of the commutation circuit. U dc And the above-mentioned second current calculation formula is calculated, and then the judgment of step S70 can be performed according to the fault current.

[0144] In this embodiment, the fault current is calculated based on the network superposition during DC fault crossing. There is no need to perform complex switching timing conversion on the initial network. It is only necessary to superimpose the calculation results of the additional network on the initial network at the switching moment. The superposition algorithm simplifies the current calculation at different stages after the fault, and while meeting the engineering accuracy requirements, it significantly improves the calculation efficiency, making this method suitable for rapid analysis and protection setting of power grid faults. Moreover, when the system contains multiple converters for DC fault crossing, it can be equivalent to superimposing multiple additional networks for DC faults. The above-mentioned method of calculating the fault current based on the network superposition during DC fault crossing is still applicable.

[0145] Furthermore, the fault current may include a DC positive current I dP and DC negative current I dN .

[0146] In a specific embodiment, the third preset condition may include a DC positive current I dP The value is less than or equal to the second current setting value I d.set2 , and the DC negative current I dN The value is less than or equal to the second current setting value I d.set2 , the second current setting value I d.set2 Determined based on the rated current of the hybrid MMC.

[0147] It should be noted that, based on the foregoing detailed description, normal operation of the hybrid MMC is restored when the third preset condition is met. Therefore, the third preset condition can be set in combination with the error range and the rated current. For example, the third preset condition is based on error setting to ensure that normal operation of the hybrid MMC can be restored when the fault current varies within a certain error range.

[0148] Specifically, the third preset condition can be ,in, , I n Indicates the rated current, k is a preset coefficient, which is determined according to the actual implementation and the possible errors in the calculation. Step S70 may include determining whether the current If so, execute step S80; otherwise, execute step S30.

[0149] The fault handling method provided in this embodiment simplifies the calculation of the fault current within the allowable range of the engineering setting error, thereby avoiding the use of electromagnetic transient simulation software for complex modeling and time-consuming calculations.

[0150] For example, in order to help understand the implementation process of the fault handling method of the embodiment of the present application, refer to Figure 6 , Figure 6 A flowchart of an application example of a troubleshooting method is provided. Specifically:

[0151] Regarding the control strategy for flexible DC transmission systems with multi-terminal hybrid MMCs, a non-lockout fault ride-through strategy can quickly clear fault currents while the converter is under control. However, this strategy is not foolproof and may still fail. As the most direct and reliable means of fault clearing, a lockout control strategy can serve as a backup protection scheme, ensuring the converter safely and reliably rides through DC faults. In light of this, a hybrid fault handling scheme combining non-lockout and lockout strategies is proposed. This scheme distinguishes fault types based on fault electrical quantity characteristics and adopts an appropriate fault strategy, thereby improving the safety and speed of the fault ride-through process, maximizing the power transmission of the DC grid, and enhancing its ability to withstand fault disturbances.

[0152] like Figure 6 As shown, first determine whether criterion 1 is met. When the DC side outlet current of the MMC reaches criterion 1, it indicates that a serious fault has occurred. Therefore, the control strategy of blocking the converter is adopted to remove the fault. If criterion 1 is not met, jump to the next step. Here, criterion 1 is based on the inter-pole short circuit fault setting, specifically ;

[0153] Further judge whether criterion 2 is met. When the DC side outlet current of a certain MMC reaches criterion 2, a non-lockout fault ride-through strategy is used to reduce the voltage and limit the current of the MMC based on negative DC voltage control to achieve fault ride-through. If criterion 2 is not met, it is considered to be a small fluctuation and normal operation can continue or the fault handling process can be terminated directly. Criterion 2 here is based on single-pole grounding fault setting, specifically: ;

[0154] During the fault ride-through process, that is, after the fault current limiting is performed, a certain delay can be made before executing subsequent judgments;

[0155] Next, determine whether criterion 3 is met. When the DC side outlet current of a certain MMC reaches criterion 3, the negative DC voltage control is switched to zero DC voltage control to avoid reverse current shock. At this time, the MMC switches to normal working conditions and establishes DC voltage. If criterion 3 is not met, it is considered that the DC fault is a permanent fault or the fault-through current limiting fails. The control strategy of the locked converter can be used to achieve fault isolation. Criterion 3 here is based on error setting, specifically: , ;

[0156] Then, it can be determined whether the fault has been cleared. When the system recognizes that the fault has been cleared, it means that the non-locked current limiting is successful, the DC fault is a transient fault, each MMC returns to normal control status, the system restores line active power transmission, and the system returns to normal operation; when the system recognizes that the fault has not been cleared, it means that the DC fault is a permanent fault. At this time, the fault line can be disconnected under zero current conditions. Specifically, the circuit breaker on the AC side of the MMC and the isolating switch on the DC side can be disconnected to isolate the fault. To restore power supply later, each MMC needs to be restarted.

[0157] In this specific application example, a superposition algorithm is used to simplify the calculation method of current at different stages after a multi-terminal DC system fault. While meeting engineering accuracy requirements, the calculation efficiency is significantly improved. The proposed method is suitable for rapid analysis and protection setting of power grid faults. Based on the post-fault electrical quantity characteristics such as voltage and current, different control strategies are dynamically matched to achieve rapid blocking and isolation of severe faults, as well as rapid recovery from transient faults, thereby improving the reliability and safety of the flexible DC system.

[0158] It should be noted that the above examples are only used to understand the present application and do not constitute a limitation on the fault handling method of the present application. More simple transformations based on this technical concept are all within the scope of protection of the present application.

[0159] The present 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 that can be executed 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 embodiment.

[0160] Reference below Figure 7 , Figure 7 The following is a schematic diagram of the hardware structure of a fault handling device suitable for implementing the embodiments of the present application. The fault handling device may include, but is not limited to, fixed terminals such as servers, network terminals, embedded computers, industrial computers, desktop computers, personal computers (PCs), digital TVs, and the like. Figure 7 The fault handling device shown is merely an example and should not limit the functions and scope of use of the embodiments of the present application.

[0161] like Figure 7 As shown, the fault handling device may include a processing device 1001 (e.g., a central processing unit, a graphics processing unit, etc.), which can perform various appropriate actions and processes based on programs stored in a read-only memory (ROM) 1002 or programs loaded from a storage device 1003 into a random access memory (RAM) 1004. RAM 1004 also stores various programs and data required for the operation of the fault handling device. Processing device 1001, ROM 1002, and RAM 1004 are interconnected via a bus 1005. An input / output (I / O) interface 1006 is also connected to bus 1005. Typically, the following systems may be connected to I / O interface 1006: input devices 1007 including, for example, a touchscreen, touchpad, keyboard, mouse, image sensor, microphone, accelerometer, gyroscope, etc.; output devices 1008 including, for example, a liquid crystal display (LCD), speaker, vibrator, etc.; storage device 1003 including, for example, a magnetic tape or hard disk; and communication device 1009. Communication device 1009 can allow the fault handling device to communicate with other devices wirelessly or wired to exchange data. Although the figure shows a fault handling device with various systems, it should be understood that it is not required to implement or have all the systems shown, and more or fewer systems can be implemented or have instead.

[0162] In particular, according to the embodiments disclosed in the present application, the processes described above with reference to the flowcharts can be implemented as computer software programs. For example, the embodiments disclosed in the present application include a computer program product comprising a computer program carried on a computer-readable storage medium, the computer program comprising a program code for executing the method shown in the flowchart. In such an embodiment, the computer program can 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 embodiment of the present application are executed.

[0163] The fault handling device provided in this application, utilizing the fault handling method described in the aforementioned embodiments, can address the technical issues in related technologies related to time-consuming and complex control processes associated with fault isolation methods for flexible DC transmission systems. Compared to related technologies, the beneficial effects of the fault handling device provided in this application are the same as those of the fault handling method described in the aforementioned embodiments. Other technical features of the fault handling device are the same as those disclosed in the fault handling method described in the aforementioned embodiments and are not further elaborated here.

[0164] It should be understood that the various parts disclosed in this application can be implemented using hardware, software, firmware, or a combination thereof. In the description of the above embodiments, specific features, structures, materials, or characteristics can be combined in any one or more embodiments or examples in a suitable manner.

[0165] The above are only specific embodiments of the present application, but the scope of protection of this application is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in this application should be included in the scope of protection of this application. Therefore, the scope of protection of this application should be based on the scope of protection of the claims.

[0166] This application also provides a flexible DC system, referring to Figure 8 , Figure 8 A connection diagram of a flexible DC system is provided. The flexible DC system may include interconnected hybrid modular multilevel converters (MMCs) and the above-mentioned fault handling device.

[0167] like Figure 8 As shown, the hybrid MMC may include a circuit breaker, a commutation circuit, a reactor, and a disconnector connected in sequence. The commutation circuit may be connected to a fault handling device, which performs the aforementioned fault handling method. The circuit breaker and disconnector may also be connected to the fault handling device, which controls their on / off. Furthermore, the disconnector is connected to a back-end power grid. Further implementations of the above components are described in the aforementioned embodiments and are not further detailed here.

[0168] The flexible DC system provided in this application, employing the fault handling method described in the aforementioned embodiments, can address the technical issues inherent in the time-consuming and complex control processes inherent in fault isolation methods used in related technologies for flexible DC transmission systems. Compared to related technologies, the flexible DC system provided in this application achieves the same beneficial effects as the fault handling method described in the aforementioned embodiments. Other technical features of this flexible DC system are the same as those disclosed in the fault handling method described in the aforementioned embodiments and are not further elaborated here.

[0169] The present application also provides a computer-readable storage medium having computer-readable program instructions (ie, a computer program) stored thereon, wherein the computer-readable program instructions are used to execute the fault handling method in the above embodiment.

[0170] The computer-readable storage medium provided herein may be, for example, a USB flash drive, but is not limited to electrical, magnetic, optical, electromagnetic, infrared, or semiconductor systems or devices, or any combination thereof. More specific examples of computer-readable storage media may include, but are not limited to, a portable computer disk electrically connected with one or more wires, a hard disk, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), optical fiber, a portable compact disk read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or the like, or any suitable combination thereof. In this embodiment, the computer-readable storage medium may be any tangible medium that contains or stores a program that can be used by an instruction execution system or device, or a combination thereof. The program code contained on the computer-readable storage medium may be transmitted using any suitable medium, including, but not limited to, wires, optical cables, RF (Radio Frequency), or the like, or any suitable combination thereof.

[0171] The computer-readable storage medium may be included in the fault handling device, or may exist independently without being assembled into the fault handling device.

[0172] The above-mentioned computer-readable storage medium carries one or more programs. When the above-mentioned one or more programs are executed by the fault handling device, the fault handling device can implement the above-mentioned functions defined in the fault handling method disclosed in the embodiment of the present application.

[0173] Computer program code for performing the operations of the present application may be written in one or more programming languages, or a combination thereof, including object-oriented programming languages ​​such as Java, Smalltalk, and C++, as well as conventional procedural programming languages ​​such as "C" or similar programming languages. The program code may be executed entirely on the user's computer, partially on the user's computer, as a stand-alone 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 may be connected to the user's computer via any type of network, including a local area network (LAN) or a wide area network (WAN), or to an external computer, such as an Internet connection provided by an Internet service provider.

[0174] The flowcharts and block diagrams in the accompanying drawings illustrate the possible architecture, functions and operations of the methods, devices, systems and computer program products according to various embodiments of the present application. In this regard, each box in the flowchart or block diagram can represent a module, program segment or a part of code, and the module, program segment or a part of code contains one or more executable instructions for realizing the specified logical function. It should also be noted that in some alternative implementations, the functions marked in the box can also occur in a different order than that marked in the accompanying drawings. For example, two boxes represented in succession can actually be executed substantially in parallel, and they can sometimes be executed in the opposite order, depending on the functions involved. It should also be noted that each box in the block diagram and / or flowchart, and the combination of the boxes in the block diagram and / or flowchart, can be implemented by a dedicated hardware-based system that performs the specified function or operation, or can be implemented by a combination of dedicated hardware and computer instructions.

[0175] The modules described in the embodiments of the present application may be implemented in software or hardware, wherein the name of a module does not necessarily limit the unit itself.

[0176] The storage medium provided in this application is a computer-readable storage medium that stores computer-readable program instructions (i.e., a computer program) for executing the aforementioned fault handling method. This computer-readable storage medium can address the technical issues in related art related to the time-consuming and complex control processes associated with fault isolation in flexible DC transmission systems. Compared to related art, the beneficial effects of the computer-readable storage medium provided in this application are similar to those of the fault handling method provided in the aforementioned embodiments and are not further elaborated here.

[0177] The above are only some embodiments of the present application and are not intended to limit the patent scope of the present application. All equivalent structural transformations made using the contents of the present application specification and drawings under the technical concept of the present application, or direct / indirect application 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 is detected in the hybrid modular multilevel converter, a fault current and a fault voltage on the DC side of the hybrid modular multilevel converter are acquired in real time; the fault current includes a DC positive current and a DC negative current, and the fault voltage includes a DC positive instantaneous voltage and a DC negative instantaneous voltage; determining whether a first preset condition is satisfied based on the fault current and the fault voltage; the first preset condition being 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 a 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 a first current setting value; If the first preset condition is met, the hybrid modular multilevel converter is locked to achieve fault isolation; otherwise, whether a second preset condition is met is further determined based on the fault voltage; 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 a second voltage setting value; If the second preset condition is met, 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, determining whether a third preset condition is satisfied based on the fault current; the third preset condition includes that the value of the DC positive current is less than or equal to a 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; If the third preset condition is met, zero voltage control is performed on the hybrid modular multilevel converter to restore normal operation of the hybrid modular multilevel converter; otherwise, blocking control is performed on the hybrid modular multilevel converter to achieve fault isolation.

2. The fault handling method according to claim 1, wherein: The hybrid modular multilevel converter includes a commutation circuit with a three-phase six-bridge-arm structure and a reactor, wherein the reactor is located on a DC output busbar of the commutation circuit; The real-time acquisition of the fault current and fault voltage on the DC side of the hybrid modular multilevel converter includes: Obtaining a DC output bus voltage of the commutation circuit; The fault current on the DC side of the hybrid modular multi-level converter is obtained according to a first current calculation formula and the DC output bus voltage; wherein the first current calculation formula is: ， in, 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; A fault voltage on the DC side of the hybrid modular multi-level converter is obtained according to the fault current and the equivalent impedance of the DC fault point.

3. The fault handling method according to claim 1, wherein: The hybrid modular multilevel converter includes 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 multiple submodules. The multiple submodules include at least one half-bridge submodule and at least one full-bridge submodule. The full-bridge submodule accounts for greater than or equal to 50% of the multiple submodules. During the fault ride-through process, determining whether a third preset condition is satisfied according to the fault current includes: During the fault ride-through process, obtaining the DC output bus voltage of the commutation circuit; The fault current on the DC side of the hybrid modular multilevel converter is obtained according to the second current calculation formula and the DC output bus voltage; wherein the second current calculation formula is: ， in, I dc is the fault current, h is the proportion of the full-bridge submodules of each bridge 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; It is determined whether a third preset condition is met according to the fault current.

4. The fault handling method according to claim 2 or 3, characterized in that: The structure of each bridge arm in the commutation circuit is consistent, and each bridge arm includes N submodules, and the N submodules include at least one half-bridge submodule and at least one full-bridge submodule, wherein the capacitance of a single capacitor in the half-bridge submodule is C 0h The capacitance of a single capacitor in the full-bridge submodule is C 0f ; The method further comprises: According to the equivalent resistance calculation formula and the bridge arm resistance of each bridge arm R arm Get the three-phase equivalent resistance R eq , the equivalent resistance calculation formula is: ， According to the equivalent inductance calculation formula and the bridge arm inductance of each bridge arm L arm Get the three-phase equivalent inductance L eq , the equivalent inductance calculation formula is: ， According to the equivalent capacitance calculation formula and the number of sub-modules N in each bridge arm, the three-phase equivalent capacitance is obtained. C eq , the equivalent capacitance calculation formula is: ， in, ε is the equivalent capacitance coefficient, which is set based on the different stages of the DC fault. C 0 =C 0h +C 0f .

5. The fault handling method according to claim 1, wherein: The performing zero voltage control on the hybrid modular multilevel converter to restore the normal operation of the hybrid modular multilevel converter includes: performing zero voltage control on the hybrid modular multilevel converter, and determining whether the DC fault is cleared; If it has been cleared, determining that the DC fault is a transient fault, and controlling the hybrid modular multilevel converter to operate normally; If not, the DC fault is determined to be a permanent fault, and the circuit breaker on the AC side and the disconnector on the DC side of the hybrid modular multilevel converter are disconnected to achieve fault isolation.

6. A fault handling device, characterized in that: The device includes a memory, a processor, and a computer program stored in the memory and running on the processor, wherein the computer program is configured to implement the steps of the fault handling method according to any one of claims 1 to 5.

7. A flexible DC system, characterized in that: The flexible DC system includes interconnected hybrid modular multilevel converters and the fault handling device according to claim 6.

8. 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. When the computer program is executed by a processor, the steps of the fault handling method according to any one of claims 1 to 5 are implemented.

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