Fault isolation and restart method and system for multi-terminal flexible direct-current transmission system

Abstract

The present application relates to the technical field of flexible direct-current transmission. Provided are a fault isolation and restart method and system for a multi-terminal flexible direct-current transmission system. The fault isolation and restart method comprises: when a single-phase grounding fault occurs at a valve side or a bridge-arm side of a multi-terminal direct-current transmission system, determining a first root mean square value of a station grounding current or a first root mean square value of a valve-side zero-sequence voltage of each converter in the multi-terminal direct-current transmission system; on the basis of the first root mean square value of the station grounding current and a second root mean square value of the station grounding current of each converter, or the first root mean square value of the valve-side zero-sequence voltage and a second root mean square value of the valve-side zero-sequence voltage of each converter, determining a faulty converter; isolating the faulty converter; and deblocking a fault-free converter in the multi-terminal direct-current transmission system, and increasing the power of the fault-free converter to a set value. The present method and system can solve the problem of simultaneous blocking of the entire multi-terminal flexible direct-current transmission system caused by a single-phase grounding fault in a single converter of the multi-terminal flexible direct-current transmission system.

Description

Fault Isolation and Restart Method and System for Multi-Terminal Flexible DC Transmission Systems Technical Field

[0001] This application relates to the field of flexible DC transmission technology, and more specifically, to a fault isolation and restart method and system for multi-terminal flexible DC transmission systems. Background Technology

[0002] Flexible direct current (DC) transmission features flexible control, rapid dynamic response, and low harmonic content. It can quickly and independently adjust active and reactive power, and possesses flexible control and coordination capabilities. It is widely considered a key technology for island power transmission, grid interconnection, reliable access to and effective utilization of new energy sources, and has broad application prospects. Especially in the field of urban grid interconnection, it can achieve reliable grid zoning, reduce short-circuit current, and enable rapid active power support between different regions.

[0003] After the deployment of multi-terminal flexible DC transmission in urban power grids, since the connection between converters and AC systems is mostly Y / Y transformers, and the transformer valve side is grounded through a resistor, if a single-phase ground fault occurs on the valve side or bridge arm side of a single converter, other non-faulty converters can also sense the fault characteristics at the same time, ultimately causing the overall blockage of multi-terminal flexible DC transmission.

[0004] Therefore, it is necessary to find a multi-terminal flexible DC back-to-back fault isolation and restart method that can distinguish and isolate faulty converters while ensuring the normal operation of other non-faulty converters. Summary of the Invention

[0005] To address at least one of the aforementioned problems, this application proposes a fault isolation and restart method and system for multi-terminal flexible DC transmission systems.

[0006] According to a first aspect of this application, at least one embodiment of this application provides a fault isolation and restart method for a multi-terminal flexible DC transmission system, comprising: in the event of a single-phase ground fault on the valve side or bridge arm side of the multi-terminal DC transmission system, determining a first effective value of the station grounding current or a first effective value of the valve side zero-sequence voltage for each converter in the multi-terminal DC transmission system; determining the faulty converter based on the first effective value and a second effective value of the station grounding current of each converter, or determining the faulty converter based on the first effective value and a second effective value of the valve side zero-sequence voltage of each converter; isolating the faulty converter; unlocking the fault-free converters in the multi-terminal DC transmission system, and increasing the power of the fault-free converters to a set value.

[0007] For example, in some embodiments of this application, determining the first effective value of the station grounding current or the first effective value of the valve-side zero-sequence voltage of each converter in the multi-terminal DC transmission system when a single-phase ground fault exists on the valve side or bridge arm side of the multi-terminal DC transmission system includes: locking each converter when a single-phase ground fault exists on the valve side or bridge arm side of the multi-terminal DC transmission system; determining the first effective value of the station grounding current, the first effective value of the valve-side zero-sequence voltage, the active power value, and the reactive power value of each converter before locking; and outputting a first AC incoming line switch command, wherein the first AC incoming line switch command is used to delay and disconnect the AC incoming line switch of each converter for a first time.

[0008] For example, in some embodiments of this application, determining the faulty converter based on the first effective value and the second effective value of the station ground current of each converter includes: determining the second effective value of the station ground current after a second blocking time for each converter, wherein the second time is less than or equal to the first time; determining that any converter has failed if the ratio of the second effective value of the station ground current to the first effective value of the station ground current of any converter in the multi-terminal DC transmission system is less than a first set threshold; and determining that any converter has not failed if the ratio of the second effective value of the station ground current to the first effective value of the station ground current of any converter in the multi-terminal DC transmission system is not less than the first set threshold.

[0009] For example, in some embodiments of this application, determining the faulty converter based on the first effective value and the second effective value of the valve-side zero-sequence voltage of each converter includes: determining the second effective value of the valve-side zero-sequence voltage after a third time period for each converter, wherein the third time period is less than or equal to the first time period; determining that any converter in the multi-terminal DC transmission system has failed if the ratio of the second effective value of the valve-side zero-sequence voltage to the first effective value of the valve-side zero-sequence voltage of any converter is less than a second set threshold; and determining that any converter has not failed if the ratio of the second effective value of the valve-side zero-sequence voltage to the first effective value of the valve-side zero-sequence voltage of any converter in the multi-terminal DC transmission system is not less than the second set threshold.

[0010] For example, in some embodiments of this application, isolating the faulty converter includes: upon determining that a faulty converter is identified, outputting a second AC input switch command, the second AC input switch command being used to immediately disconnect the AC input switch of the faulty converter; and, upon disconnecting the AC input switch of the faulty converter, outputting a command to disconnect the DC side switch and the isolator of the faulty converter.

[0011] For example, in some embodiments of this application, the multi-terminal flexible DC transmission system includes n converters, one of the n converters is designated as a DC voltage-controlled converter, and n-1 of the n converters is designated as an active power-controlled converter, where n is an integer greater than or equal to 3.

[0012] For example, in some embodiments of this application, unlocking the fault-free converter in the multi-terminal DC transmission system and increasing the power of the fault-free converter to a set value includes: when the faulty converter is determined, outputting a cancellation command, the cancellation command being used to cancel the first AC incoming line switch command; when the DC side switch of the faulty converter is open, determining the type of the faulty converter; when the faulty converter is an active power control operating converter, unlocking the DC voltage control operating converter; delaying for a fourth time to unlock the fault-free active power control operating converter; restoring the active power value of the fault-free active power control operating converter to a first set value according to a first set rate; and restoring the reactive power value of the fault-free active power control operating converter to a second set value according to a second set rate.

[0013] For example, in some embodiments of this application, the step of unlocking the faultless converter in the multi-terminal DC transmission system and increasing the power of the faultless converter to a set value further includes: when the faulty converter is the DC voltage control operation converter, according to the set DC voltage station takeover sequence, setting the active power control operation converter with the highest takeover priority among the faultless active power control operation converters as the DC voltage control operation converter and unlocking it; delaying for a fifth time to unlock the faultless active power control operation converter; restoring the active power value of the faultless active power control operation converter to a third set value according to a third set rate; and restoring the reactive power value of the faultless active power control operation converter to a fourth set value according to a fourth set rate.

[0014] For example, in some embodiments of this application, the first set value and the third set value include: active power value determined before locking and / or active power value determined according to active power demand; the second set value and the fourth set value include: reactive power value determined before locking and / or reactive power value determined according to reactive power demand.

[0015] For example, in some embodiments of this application, the converter in the multi-terminal flexible DC transmission system is connected to the AC system via a Y / Y transformer, and the valve side of the transformer is grounded via a resistor.

[0016] According to a second aspect of this application, at least one embodiment of this application provides a fault isolation and restart system for a multi-terminal flexible DC transmission system, used to execute the fault isolation and restart method as described in any one of the first aspects. The fault isolation and restart system includes: a fault detection module, used to determine a first effective value of the station grounding current or a first effective value of the valve-side zero-sequence voltage of each converter in the multi-terminal DC transmission system when a single-phase grounding fault exists on the valve side or bridge arm side of the multi-terminal DC transmission system; a fault converter determination module, used to determine the faulty converter based on the first effective value and a second effective value of the station grounding current of each converter, or based on the first effective value and a second effective value of the valve-side zero-sequence voltage of each converter; a faulty converter isolation module, used to isolate the faulty converter; and a non-faulty converter restart module, used to unlock the fault-free converters in the multi-terminal DC transmission system and increase the power of the fault-free converters to a set value.

[0017] Through the above-described examples and embodiments, this application provides a fault isolation and restart method and system for multi-terminal flexible DC transmission systems. By detecting and comparing the magnitude of the station grounding current before and after blocking, and recording the active power, reactive power, and operating mode before blocking, the system identifies and isolates faulty converters and restarts non-faulty converters. This avoids the overall operation of multi-terminal flexible DC transmission systems caused by a single converter fault, solves the problem that a single-phase grounding fault on the valve side or bridge arm side of a single converter in a multi-terminal flexible DC transmission system can cause the entire multi-terminal flexible DC transmission system to be blocked simultaneously, and improves the operational reliability of multi-terminal flexible DC transmission systems.

[0018] It should be understood that the above general description and the following detailed description are merely exemplary and do not limit this application. Attached Figure Description

[0019] The above and other objects, features, and advantages of this application will become more apparent from the detailed description of exemplary embodiments with reference to the accompanying drawings. The drawings described below are merely some embodiments of this application and are not intended to limit the scope of this application.

[0020] Figure 1 shows a schematic diagram of the multi-terminal flexible DC transmission system of this application;

[0021] Figure 2 shows a schematic diagram of the topology of a single converter in this application;

[0022] Figure 3 shows a flowchart of a fault isolation and restart method for a multi-terminal flexible DC transmission system according to an exemplary embodiment;

[0023] Figure 4 shows a schematic diagram of a fault isolation and restart system for a multi-terminal flexible DC transmission system, according to an exemplary embodiment. Embodiments of the present invention

[0024] Exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, these exemplary embodiments can be implemented in many forms and should not be construed as limited to the embodiments set forth herein; rather, they are provided so that this application will be thorough and complete, and will fully convey the concept of the exemplary embodiments to those skilled in the art. The same reference numerals in the drawings denote the same or similar parts, and therefore repeated descriptions of them will be omitted.

[0025] The described features, structures, or characteristics can be combined in any suitable manner in one or more embodiments. Numerous specific details are provided in the following description to give a full understanding of embodiments of this disclosure. However, those skilled in the art will recognize that the technical solutions of this disclosure can be practiced without one or more of these specific details, or other methods, components, materials, devices, etc. In these cases, well-known structures, methods, devices, implementations, materials, or operations will not be shown or described in detail.

[0026] The flowcharts shown in the accompanying drawings are merely illustrative and do not necessarily include all content and operations / steps, nor do they necessarily have to be performed in the described order. For example, some operations / steps can be broken down, while others can be combined or partially combined; therefore, the actual execution order may change depending on the specific circumstances.

[0027] The terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish different objects, not to describe a specific order. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products, or apparatuses.

[0028] Those skilled in the art will understand that the accompanying drawings are merely schematic diagrams of exemplary embodiments, and the modules or processes in the drawings are not necessarily essential for implementing this application, and therefore cannot be used to limit the scope of protection of this application.

[0029] Figure 1 shows a schematic diagram of the multi-terminal flexible DC transmission system of this application.

[0030] As shown in Figure 1, the multi-terminal flexible DC transmission system includes n converters: one of the n converters is designated as a DC voltage-controlled converter, and n-1 of the n converters are designated as active power-controlled converters, where n is an integer greater than or equal to 3. Converter 1 is designated as the DC voltage-controlled converter, and the other converters are designated as active power-controlled converters. A running converter refers to a converter that is in the unlocked state and connected to the DC side.

[0031] Figure 2 shows a schematic diagram of the topology of a single converter in this application.

[0032] As shown in Figure 2, each converter includes an AC incoming switch QF1, DC side switches Q1, Q91, Q2, Q92, and disconnectors Q51, Q52, Q53, Q54, Q55, Q56.

[0033] In a multi-terminal flexible DC transmission system, the converter is connected to the AC system via a Y / Y transformer, and the valve side of the transformer is grounded via a resistor.

[0034] Figure 3 shows a flowchart of a fault isolation and restart method for a multi-terminal flexible DC transmission system according to an exemplary embodiment.

[0035] As shown in Figure 3, the steps of the fault isolation and restart method for multi-terminal flexible DC transmission systems include:

[0036] Step S101: In the event of a single-phase ground fault on the valve side or bridge arm side of the multi-terminal DC transmission system, determine the first effective value of the station grounding current or the first effective value of the valve side zero-sequence voltage for each converter in the multi-terminal DC transmission system.

[0037] According to the example embodiment, the effective value of the station grounding current of the operating converter is detected in real time. In the event of a single-phase ground fault on the valve side or bridge arm side of a multi-terminal DC transmission system, each converter in the system is locked out. The first effective value of the station grounding current for each converter in the multi-terminal DC transmission system before locking out is determined. The first effective value of the zero-sequence voltage on the valve side, U 01 The system outputs active power value P and reactive power value Q, and outputs a first AC incoming line switch command, which is used to delay the first time. Disconnect the AC incoming line switch of each converter in a multi-terminal DC transmission system.

[0038] For example, record the effective value of the station grounding current before blocking. The active power value P and reactive power value Q are measured 50ms before the blocking. This can be set manually, for example... .

[0039] Step S102: Determine the faulty converter based on the first effective value and the second effective value of the station grounding current of each converter, or based on the first effective value and the second effective value of the valve-side zero-sequence voltage of each converter.

[0040] According to the example embodiment, after the converter is locked out, the effective value of the grounding current at the monitoring station is determined. The second lockout time for each converter in the multi-terminal DC transmission system is determined. The second effective value of the subsequent station grounding current Among them, the second time Less than or equal to the first time , .

[0041] If the ratio of the second effective value of the station grounding current to the first effective value of the station grounding current of any converter in a multi-terminal DC transmission system is less than a first preset threshold, it is determined that the current converter has failed. If the ratio of the second effective value of the station grounding current to the first effective value of the station grounding current of any converter in a multi-terminal DC transmission system is not less than the first preset threshold, it is determined that the current converter has not failed.

[0042] For example, after the converter is locked out, the effective value of the grounding current at the monitoring station. and delay Subsequent grounding current locked at the time of blocking If a comparison is made, If so, the converter is determined to be faulty, and the fault flag bit VF=1 is set. If the condition is met, it is determined that the converter has not malfunctioned, and the fault flag bit VF is set to 0. The first set threshold can be set by the user; this application only uses 0.5 as an example, but this application is not limited to this.

[0043] According to the example embodiment, after the converter is locked out, the effective value of the zero-sequence voltage on the valve side is detected. The third lockout time for each converter in the multi-terminal DC transmission system is determined. The second effective value of the zero-sequence voltage U on the valve side after 02 Among them, the third time Less than or equal to the first time , .

[0044] If the ratio of the second effective value of the valve-side zero-sequence voltage to the first effective value of the valve-side zero-sequence voltage of any converter in the multi-terminal DC transmission system is less than a second set threshold, it is determined that any converter has failed; if the ratio of the second effective value of the valve-side zero-sequence voltage to the first effective value of the valve-side zero-sequence voltage of any converter in the multi-terminal DC transmission system is not less than the second set threshold, it is determined that any converter has not failed.

[0045] For example, after the converter is locked out, the effective value of the zero-sequence voltage U on the detection valve side is measured. 02 and delay The effective value of the zero-sequence voltage U on the valve side locked at the time of interlocking. 01 If a comparison is made, If so, the converter is determined to be faulty, and the fault flag bit VF=1 is set. If the condition is met, it is determined that the converter has not malfunctioned, and the fault flag bit VF is set to 0. The second threshold can be set by the user; this application uses 0.5 as an example, but it is not limited to this.

[0046] Step S103: Isolate the faulty converter.

[0047] According to the example embodiment, upon determining that a faulty converter has been identified, a second AC input switch command is output. This second AC input switch command is used to immediately disconnect the AC input switch of the faulty converter. If the AC input switch of the faulty converter is disconnected, a command is output to disconnect the DC-side switch and the isolator of the faulty converter.

[0048] Step S104: Unlock the fault-free converter in the multi-terminal DC transmission system and increase the power of the fault-free converter to the set value.

[0049] According to the example embodiment, if a faulty converter is identified, a cancellation command is output to cancel the first AC input switch command. If the DC-side switch of the faulty converter is open, the type of faulty converter is determined:

[0050] If the faulty converter is an active power control converter, unlock the DC voltage control converter; after a fourth delay, unlock the fault-free active power control converter; restore the active power value of the fault-free active power control converter to the first set value according to the first set rate; restore the reactive power value of the fault-free active power control converter to the second set value according to the second set rate.

[0051] When the faulty converter is a DC voltage control operating converter, according to the set DC voltage station takeover sequence, the active power control operating converter with the highest takeover priority among the fault-free active power control operating converters is set as a DC voltage control operating converter and unlocked; the fault-free active power control operating converter is unlocked after a fifth delay; the active power value of the fault-free active power control operating converter is restored to the third set value according to the third set rate; the reactive power value of the fault-free active power control operating converter is restored to the fourth set value according to the fourth set rate.

[0052] For example, if it is determined that the current converter is a non-faulty converter and the fault flag bit VF=1 is received from other converters, a cancellation command is immediately output to cancel the delay. The instruction to disconnect the AC incoming switch of each converter in the multi-terminal DC transmission system is given, and the DC side switch of the faulty converter is allowed to open.

[0053] After the DC-side switch of the faulty converter is opened, if the faulty converter is an active power control operating converter, the DC voltage control operating converters in other converters with fault flag VF=0 will be unlocked first. After a delay of 50ms, the other fault-free active power control operating converters will be unlocked, and the active power value of the active power control operating converters will be restored to the set active power value at a certain rate. The reactive power value of the active power control operating converters will also be restored to the set reactive power value at a certain rate.

[0054] For example, if the faulty converter is a DC voltage control operating converter, according to the set DC voltage station takeover sequence, the active power control operating converter with the highest priority in the DC voltage station takeover sequence among the other converters with fault flag VF=0 is taken as a DC voltage control operating converter and unlocked. After a delay of 50ms, the other fault-free active power control operating converters are unlocked, and the active power value of the active power control operating converter is restored to the set active power value at a certain rate, and the reactive power value of the active power control operating converter is restored to the set reactive power value at a certain rate.

[0055] According to some embodiments, the first and third set values ​​include: active power values ​​determined before locking and / or active power values ​​determined based on active power demand. The second and fourth set values ​​include: reactive power values ​​determined before locking and / or reactive power values ​​determined based on reactive power demand.

[0056] This application also provides a fault isolation and restart system for a multi-terminal flexible DC transmission system, used to execute the fault isolation and restart method for a multi-terminal flexible DC transmission system as described above. As shown in Figure 4, the fault isolation and restart system includes: a fault detection module, a fault converter judgment module, a fault converter isolation module, and a non-fault converter restart module.

[0057] The fault detection module is used to determine the first effective value of the station grounding current or the first effective value of the valve-side zero-sequence voltage of each converter in the multi-terminal DC transmission system when a single-phase grounding fault exists on the valve side or bridge arm side.

[0058] The fault converter determination module is used to determine the faulty converter based on the first effective value and the second effective value of the station grounding current of each converter, or based on the first effective value and the second effective value of the valve-side zero-sequence voltage of each converter.

[0059] The fault converter isolation module is used to isolate faulty converters.

[0060] The non-faulty converter restart module is used to unlock fault-free converters in a multi-terminal DC transmission system and increase the power of the fault-free converters to a set value.

[0061] The fault isolation and restart system is used to perform the fault isolation and restart methods described above, so it will not be repeated here.

[0062] This application provides a fault isolation and restart method and system for multi-terminal flexible DC transmission systems. By detecting and comparing the grounding current values ​​of stations before and after blocking, and recording the active power, reactive power, and operating mode before blocking, the system identifies and isolates faulty converters and restarts non-faulty converters. This avoids the overall operation of multi-terminal flexible DC transmission systems caused by a single converter fault, and solves the problem that a single-phase grounding fault on the valve side or bridge arm side of a single converter in a multi-terminal flexible DC transmission system can cause the entire multi-terminal flexible DC transmission system to be blocked simultaneously, thereby improving the operational reliability of the multi-terminal flexible DC transmission system.

[0063] It should be clearly understood that this application describes how specific examples are formed and used, but this application is not limited to any details of these examples. Rather, based on the teachings of the disclosure of this application, these principles can be applied to many other embodiments.

[0064] Furthermore, it should be noted that the above figures are merely illustrative representations of the processes included in the method according to exemplary embodiments of this application, and are not intended to be limiting. It is readily understood that the processes shown in the above figures do not indicate or limit the temporal order of these processes. Additionally, it is readily understood that these processes may be executed synchronously or asynchronously, for example, in multiple modules.

[0065] Exemplary embodiments of this application have been specifically shown and described above. It should be understood that this application is not limited to the detailed structures, arrangements, or implementation methods described herein; rather, this application is intended to cover various modifications and equivalent arrangements that fall within the objectives and scope of the appended claims.

Claims

1. A fault isolation and restart method for multi-terminal flexible DC transmission systems, characterized in that, include: In the event of a single-phase ground fault on the valve side or bridge arm side of the multi-terminal DC transmission system, determine the first effective value of the station grounding current or the first effective value of the valve side zero-sequence voltage for each converter in the multi-terminal DC transmission system. The faulty converter is determined based on the first effective value and the second effective value of the station grounding current of each converter, or based on the first effective value and the second effective value of the valve-side zero-sequence voltage of each converter. Isolate the converter that is experiencing the fault; Unlock the fault-free converter in the multi-terminal DC transmission system and increase the power of the fault-free converter to a set value.

2. The fault isolation and restart method as described in claim 1, characterized in that, In the event of a single-phase ground fault on the valve side or bridge arm side of the multi-terminal DC transmission system, determining the first effective value of the station grounding current or the first effective value of the valve side zero-sequence voltage for each converter in the multi-terminal DC transmission system includes: In the event of a single-phase ground fault on the valve side or bridge arm side of the multi-terminal DC transmission system, each converter shall be locked out. Determine the first effective value of the station grounding current, the first effective value of the valve-side zero-sequence voltage, the active power value, and the reactive power value of each converter before the blocking; Output a first AC input switch instruction, which is used to delay and disconnect the AC input switch of each converter at a first time.

3. The fault isolation and restart method as described in claim 2, characterized in that, The step of determining the faulty converter based on the first effective value and the second effective value of the station grounding current of each converter includes: Determine the second effective value of the station grounding current after the second time of blocking each converter, wherein the second time is less than or equal to the first time; If the ratio of the second effective value of the station grounding current to the first effective value of the station grounding current of any converter in the multi-terminal DC transmission system is less than a first set threshold, it is determined that any converter has failed. If the ratio of the second effective value of the station grounding current to the first effective value of the station grounding current of any converter in the multi-terminal DC transmission system is not less than a first set threshold, it is determined that the converter has not failed.

4. The fault isolation and restart method as described in claim 2, characterized in that, The step of determining the faulty converter based on the first effective value and the second effective value of the valve-side zero-sequence voltage of each converter includes: Determine the second effective value of the valve-side zero-sequence voltage after the third time of lock-up for each converter, wherein the third time is less than or equal to the first time; If the ratio of the second effective value of the valve-side zero-sequence voltage to the first effective value of the valve-side zero-sequence voltage of any converter in the multi-terminal DC transmission system is less than a second set threshold, it is determined that any converter has failed. If the ratio of the second effective value of the valve-side zero-sequence voltage to the first effective value of the valve-side zero-sequence voltage of any converter in the multi-terminal DC transmission system is not less than a second set threshold, it is determined that the converter has not failed.

5. The fault isolation and restart method as described in claim 3, characterized in that, The converter that isolates the fault includes: In the event that a faulty converter is identified, a second AC input switch command is output, which is used to immediately disconnect the AC input switch of the faulty converter. When the AC input switch of the faulty converter is disconnected, an instruction is output to disconnect the DC side switch and the isolator of the faulty converter.

6. The fault isolation and restart method as described in claim 5, characterized in that, The multi-terminal flexible DC transmission system includes n converters, one of which is designated as a DC voltage-controlled converter, and n-1 of which are designated as active power-controlled converters, where n is an integer greater than or equal to 3.

7. The fault isolation and restart method as described in claim 6, characterized in that, Unlocking the fault-free converter in the multi-terminal DC transmission system and increasing the power of the fault-free converter to a set value includes: If the faulty converter is identified, a cancellation command is output, which is used to cancel the first AC incoming line switch command; If the DC-side switch of the faulty converter is disconnected, determine the type of the faulty converter. If the faulty converter is an active power control converter, unlock the DC voltage control converter. The active power control converter is unlocked after a fourth delay to ensure it operates without faults. The active power value of the fault-free active power control converter is restored to the first set value according to the first set rate. The reactive power value of the fault-free active power control converter is restored to the second set value according to the second set rate.

8. The fault isolation and restart method as described in claim 7, characterized in that, The step of unlocking the fault-free converter in the multi-terminal DC transmission system and increasing the power of the fault-free converter to a set value also includes: If the faulty converter is the DC voltage control operation converter, according to the set DC voltage station takeover sequence, the active power control operation converter with the highest takeover priority among the fault-free active power control operation converters is set as the DC voltage control operation converter and unlocked. The active power control converter is unlocked after a fifth delay and is in operation without faults. The active power value of the fault-free active power control converter is restored to the third set value according to the third set rate. The reactive power value of the fault-free active power control converter is restored to the fourth set value according to the fourth set rate.

9. The fault isolation and restart method as described in claim 8, characterized in that, The first set value and the third set value include: the active power value determined before locking and / or the active power value determined according to the active power demand; The second setting value and the fourth setting value include: the reactive power value determined before locking and / or the reactive power value determined according to the reactive power demand.

10. The fault isolation and restart method as described in claim 1, characterized in that, The converter in the multi-terminal flexible DC transmission system is connected to the AC system via a Y / Y transformer, and the valve side of the transformer is grounded via a resistor.

11. A fault isolation and restart system for a multi-terminal flexible DC transmission system, characterized in that, For performing the fault isolation and restart method as described in any one of claims 1-10, the fault isolation and restart system comprises: The fault detection module is used to determine the first effective value of the station grounding current or the first effective value of the valve-side zero-sequence voltage of each converter in the multi-terminal DC transmission system when a single-phase grounding fault exists on the valve side or bridge arm side of the multi-terminal DC transmission system. The fault converter judgment module is used to determine the faulty converter based on the first effective value and the second effective value of the station grounding current of each converter, or based on the first effective value and the second effective value of the valve side zero-sequence voltage of each converter. A faulty converter isolation module is used to isolate the faulty converter. The non-faulty converter restart module is used to unlock the faultless converter in the multi-terminal DC transmission system and increase the power of the faultless converter to a set value.

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