Ground fault protection method, device and equipment for converter of direct-current power transmission system

Abstract

The invention discloses a DC power transmission system converter grounding fault protection method, device and equipment, which are applied to a bipolar flexible DC power transmission system. The method comprises the following steps: acquiring a first characteristic quantity and a second characteristic quantity of a small current grounding fault of the converter; wherein the first characteristic quantity of the small-current grounding fault of the current converter is obtained according to the direct current of a high-voltage bus and the direct current of a grounding electrode bus of the current converter, and the second characteristic quantity of the small-current grounding fault of the current converter is obtained according to the alternating current three-phase current of the valve side of the current converter; and according to the first characteristic quantity and the second characteristic quantity of the small-current grounding fault of the current converter, judging that the small-current grounding fault of the current converter occurs. According to the invention, whether the converter has a grounding fault can be simply, reliably and quickly judged, and the fault is convenient to overhaul and clear.

Description

Technical Field

[0001] The present invention belongs to the technical field of flexible direct current transmission, and in particular relates to a method, device and equipment for protecting a converter from ground fault in a direct current transmission system. Background Art

[0002] When the flexible DC transmission system is in bipolar balanced operation mode, a ground fault in the valve area of ​​the converter close to the neutral line will generate a small fault current. Although it is not enough to trigger the protection configured in the flexible DC transmission system, if the fault is not cleared, it may develop into a more serious fault, causing damage to the converter valve and bridge arm reactor. In the current research on fault protection of DC transmission systems, the identification and protection of small current ground faults in converters are rarely considered.

[0003] The closest patent number retrieved is CN111555248A, and the patent name is "A method and device for protecting the ground fault of the converter in a flexible DC power transmission system". This invention can effectively detect the ground fault in the connection area between the flexible DC converter and the pole-neutral bus by comparing the current difference between the pole-bus converter side current and the pole-neutral bus converter side current with the current constant value. However, this invention only uses the difference between the two-pole bus currents as a criterion, which is prone to misjudgment, and the operator needs to further determine the area that needs maintenance. Summary of the invention

[0004] The purpose of the present invention is to provide a method, device and equipment for protecting a converter from ground fault in a DC power transmission system, which can simply, reliably and quickly determine whether a converter has a ground fault, thereby facilitating the inspection and removal of the fault.

[0005] In order to achieve the above object, the solution of the present invention is:

[0006] A method for protecting a converter from ground fault in a direct current transmission system, applied to a bipolar flexible direct current transmission system; comprising:

[0007] Obtain a first characteristic value of a small current grounding fault of a converter and a second characteristic value of a small current grounding fault of a converter; wherein the first characteristic value of a small current grounding fault of a converter is obtained according to a DC current of a high-voltage bus of a converter and a DC current of a grounding bus, and the second characteristic value of a small current grounding fault of a converter is obtained according to an AC three-phase current on a valve side of the converter;

[0008] According to the first characteristic value and the second characteristic value of the converter low current grounding fault, it is determined that a converter low current grounding fault occurs.

[0009] The first characteristic quantity of the converter low current grounding fault is obtained according to the converter high voltage bus DC current and the grounding electrode bus DC current, and includes:

[0010] Obtaining the DC current of the converter high voltage bus and the DC current of the grounding electrode bus;

[0011] The difference between the DC current of the high-voltage busbar of the converter and the DC current of the grounding electrode busbar is obtained, and the effective value of the difference is used as the first characteristic quantity of the low-current grounding fault of the converter.

[0012] The second characteristic quantity of the converter small current grounding fault is obtained according to the converter valve side AC three-phase current, including:

[0013] Obtaining the AC three-phase current on the converter valve side;

[0014] A zero-sequence current of the three-phase AC current on the valve side of the converter is obtained, and the zero-sequence current is used as a second characteristic quantity of a small current grounding fault of the converter.

[0015] Wherein, judging the occurrence of a converter low current grounding fault according to the first characteristic quantity and the second characteristic quantity of the converter low current grounding fault includes:

[0016] When conditions one to four are met at the same time, it is determined that a small current grounding fault of the converter occurs;

[0017] Condition 1: The flexible DC transmission system is in bipolar operation mode;

[0018] Condition 2: the first characteristic value of the converter low-current grounding fault is greater than a set first threshold current;

[0019] Condition three: the second characteristic value of the converter low current grounding fault is greater than a set second threshold current;

[0020] Condition 4: The duration of conditions 1 to 3 being met is greater than the set delay, and no other protection actions are triggered.

[0021] Among them, in the condition 2, the first threshold current is greater than three times or more the maximum measurement error percentage of the measuring equipment in the flexible direct current transmission system in a steady state; in the condition 3, the second threshold current is greater than three times or more the maximum measurement error percentage of the measuring equipment in the flexible direct current transmission system in a steady state.

[0022] Among them, in the condition three, the second threshold current is less than the bridge arm inductor differential protection setting and the minimum triggering current of the bridge arm differential protection.

[0023] Among them, in the condition four, the set delay is simultaneously greater than the time setting values ​​of other protection actions in the flexible direct current transmission system.

[0024] Wherein, when judging that a small current grounding fault of the converter occurs, it also includes:

[0025] After a pre-delay, the converter is locked and the AC incoming line circuit breaker connected to the converter is tripped, or the local pole power is reduced after a preset delay.

[0026] Wherein, when judging that a small current grounding fault of the converter occurs, it also includes:

[0027] Send out an alarm signal.

[0028] A DC power transmission system converter grounding fault protection device, applied to a bipolar flexible DC power transmission system; comprising:

[0029] A fault collection module is configured to obtain a DC current of a high-voltage busbar of a converter, a DC current of a grounding busbar, and an AC three-phase current of a valve side of the converter;

[0030] A characteristic calculation module is configured to obtain a first characteristic value of a small current grounding fault of a converter according to a DC current of a high-voltage busbar of the converter and a DC current of a grounding electrode busbar, and to obtain a second characteristic value of a small current grounding fault of a converter according to an AC three-phase current on a valve side of the converter; and,

[0031] The fault judgment module is configured to judge whether a small current grounding fault of the converter occurs according to the first characteristic value and the second characteristic value of the small current grounding fault of the converter.

[0032] The characteristic calculation module obtains the first characteristic quantity of the converter low current grounding fault according to the converter high voltage bus DC current and the grounding electrode bus DC current, including:

[0033] Obtaining the DC current of the converter high voltage bus and the DC current of the grounding electrode bus;

[0034] The difference between the DC current of the high-voltage busbar of the converter and the DC current of the grounding electrode busbar is obtained, and the effective value of the difference is used as the first characteristic quantity of the low-current grounding fault of the converter.

[0035] The characteristic calculation module obtains the second characteristic value of the converter low current grounding fault according to the AC three-phase current on the converter valve side, including:

[0036] Obtaining the AC three-phase current on the converter valve side;

[0037] A zero-sequence current of the three-phase AC current on the valve side of the converter is obtained, and the zero-sequence current is used as a second characteristic quantity of a small current grounding fault of the converter.

[0038] The fault judgment module judges whether a small current grounding fault of the converter occurs according to the first characteristic value and the second characteristic value of the small current grounding fault of the converter, including:

[0039] When conditions one to four are met at the same time, it is determined that a small current grounding fault of the converter occurs;

[0040] Condition 1: The flexible DC transmission system is in bipolar operation mode;

[0041] Condition 2: the first characteristic value of the converter low-current grounding fault is greater than a set first threshold current;

[0042] Condition three: the second characteristic value of the converter low current grounding fault is greater than a set second threshold current;

[0043] Condition 4: The duration of conditions 1 to 3 being met is greater than the set delay, and no other protection actions are triggered.

[0044] Among them, in the condition 2, the first threshold current is greater than three times or more the maximum measurement error percentage of the measuring equipment in the flexible direct current transmission system in a steady state; in the condition 3, the second threshold current is greater than three times or more the maximum measurement error percentage of the measuring equipment in the flexible direct current transmission system in a steady state.

[0045] Among them, in the condition three, the second threshold current is less than the bridge arm inductor differential protection setting and the minimum triggering current of the bridge arm differential protection.

[0046] Among them, in the condition four, the set delay is simultaneously greater than the time setting values ​​of other protection actions in the flexible direct current transmission system.

[0047] Wherein, when the fault judgment module judges that a small current grounding fault of the converter occurs, it also includes:

[0048] After a pre-delay, the converter is locked and the AC incoming line circuit breaker connected to the converter is tripped, or the local pole power is reduced after a preset delay.

[0049] It also includes an alarm display module, which is configured to send out an alarm signal.

[0050] An electronic device comprises a memory, a processor, and a computer program stored in the memory and executable on the processor; when the processor executes the computer program, the steps of the above-mentioned method for protecting a converter from ground fault in a DC transmission system are implemented.

[0051] A computer-readable storage medium stores a computer program; when the computer program is executed by a processor, the steps of the above-mentioned method for protecting a converter from ground fault in a direct current transmission system are implemented.

[0052] After adopting the above scheme, compared with the prior art, the present invention can quickly and accurately determine whether a small current grounding fault occurs in the DC transmission system converter through dual judgment criteria, which is convenient for executing protection actions and arranging maintenance and fault clearing. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 is an overall flow chart of a method for protecting a converter from ground fault in a DC power transmission system according to an embodiment of the present invention;

[0054] Figure 2 is a bipolar topological structure diagram of a flexible DC converter provided in an embodiment of the present invention;

[0055] Figure 3 It is an overall structural diagram of a converter ground fault protection device for a DC power transmission system according to an embodiment of the present invention;

[0056] Figure 4 It is a schematic diagram of the structure of an electronic device according to an embodiment of the present invention. DETAILED DESCRIPTION

[0057] The technical solutions and beneficial effects of the present invention will be described in detail below with reference to the accompanying drawings.

[0058] The embodiment of the present invention provides a method for protecting a converter from ground fault in a DC power transmission system, which is applied to a bipolar flexible DC power transmission system; the method comprises:

[0059] Obtain a first characteristic value of a small current grounding fault of a converter and a second characteristic value of a small current grounding fault of a converter; wherein the first characteristic value of a small current grounding fault of a converter is obtained according to a DC current of a high-voltage bus of a converter and a DC current of a grounding bus, and the second characteristic value of a small current grounding fault of a converter is obtained according to an AC three-phase current on a valve side of the converter;

[0060] According to the first characteristic value and the second characteristic value of the converter low current grounding fault, it is determined that a converter low current grounding fault occurs.

[0061] The first characteristic quantity of the converter low current grounding fault is obtained according to the converter high voltage bus DC current and the grounding electrode bus DC current, and includes:

[0062] Obtaining the DC current of the converter high voltage bus and the DC current of the grounding electrode bus;

[0063] The difference between the DC current of the high-voltage busbar of the converter and the DC current of the grounding electrode busbar is obtained, and the effective value of the difference is used as the first characteristic quantity of the low-current grounding fault of the converter.

[0064] The second characteristic quantity of the converter small current grounding fault is obtained according to the converter valve side AC three-phase current, including:

[0065] Obtaining the AC three-phase current on the converter valve side;

[0066] A zero-sequence current of the three-phase AC current on the valve side of the converter is obtained, and the zero-sequence current is used as a second characteristic quantity of a small current grounding fault of the converter.

[0067] Wherein, judging the occurrence of a converter low current grounding fault according to the first characteristic quantity and the second characteristic quantity of the converter low current grounding fault includes:

[0068] When conditions one to four are met at the same time, it is determined that a small current grounding fault of the converter occurs;

[0069] Condition 1: The flexible DC transmission system is in bipolar operation mode;

[0070] Condition 2: the first characteristic value of the converter low-current grounding fault is greater than a set first threshold current;

[0071] Condition three: the second characteristic value of the converter low current grounding fault is greater than a set second threshold current;

[0072] Condition 4: The duration of conditions 1 to 3 being met is greater than the set delay, and no other protection actions are triggered.

[0073] Among them, in the condition 2, the first threshold current is greater than three times or more the maximum measurement error percentage of the measuring equipment in the flexible direct current transmission system in a steady state; in the condition 3, the second threshold current is greater than three times or more the maximum measurement error percentage of the measuring equipment in the flexible direct current transmission system in a steady state.

[0074] Among them, in the condition three, the second threshold current is less than the bridge arm inductor differential protection setting and the minimum triggering current of the bridge arm differential protection.

[0075] Among them, in the condition four, the set delay is simultaneously greater than the time setting values ​​of other protection actions in the flexible direct current transmission system.

[0076] Wherein, when judging that a small current grounding fault of the converter occurs, it also includes:

[0077] After a pre-delay, the converter is locked and the AC incoming line circuit breaker connected to the converter is tripped, or the local pole power is reduced after a preset delay.

[0078] Wherein, when judging that a small current grounding fault of the converter occurs, it also includes:

[0079] Send out an alarm signal.

[0080] See also Figure 1 , showing the overall process of the DC power transmission system converter grounding fault protection method provided by the embodiment of the present invention. The DC power transmission system converter grounding fault protection method provided by the present invention is applied to a bipolar flexible DC power transmission system, and the method comprises:

[0081] Step 101: collecting the converter high-voltage bus DC current IdH, the grounding bus DC current IdN and the valve-side AC three-phase current Ivc;

[0082] In some embodiments, the above-mentioned acquisition points can refer to Figure 2 , Figure 2 The bipolar topology structure of the flexible DC converter provided by the embodiment of the present invention is illustrated.

[0083] Step 102: Calculate a first characteristic value and a second characteristic value of a converter low current grounding fault;

[0084] In some embodiments, the first characteristic quantity of the converter's small current grounding fault is calculated by collecting the converter's high-voltage bus DC current IdH and the grounding bus's DC current IdN, and the first characteristic quantity of the converter's small current grounding fault is the effective value Idv of the difference between the converter's high-voltage bus's DC current IdH and the grounding bus's DC current IdN.

[0085] In some embodiments, the second characteristic quantity of the converter small current grounding fault is determined by the collected valve side AC three-phase current Ivc, and the second characteristic quantity of the converter small current grounding fault is the zero-sequence current Iv0 generated by the valve side three-phase current.

[0086] Step 103: Determine whether a small current grounding fault of the converter occurs;

[0087] In a preferred solution, judging whether the converter low current grounding fault occurs according to the first characteristic value and the second characteristic value of the converter low current grounding fault includes:

[0088] 1) The flexible DC system is in bipolar operation mode;

[0089] 2) The first characteristic value of the fault is greater than the set threshold current I1;

[0090] 3) The amplitude of the second characteristic value of the fault is greater than the set threshold current I2;

[0091] 4) The duration is greater than the set delay T and no other protection actions are triggered.

[0092] In some embodiments, the threshold current I1 and the threshold current I2 are three times or more greater than the maximum measurement error percentage of the measuring device in the flexible direct current transmission system in a steady state.

[0093] In some embodiments, the threshold current I2 is smaller than the bridge arm reactor differential protection setting and the minimum trigger current of the bridge arm differential protection.

[0094] In some embodiments, the set delay T should also be greater than the time settings of other protections in the flexible DC transmission system, such as AC connecting line differential protection, bridge arm differential protection, bridge arm reactor differential protection, etc.

[0095] Step 104: Send an alarm signal.

[0096] In some embodiments, after determining that a small current grounding fault of the converter occurs, it also includes fault clearing, locking the converter after a preset delay and tripping the AC incoming line circuit breaker connected to the converter, or reducing the local pole power after a preset delay.

[0097] In some embodiments, the above method can be used to effectively determine Figure 2 This includes but is not limited to small current grounding faults occurring in F1 and F2 areas, which facilitates further maintenance and fault clearing.

[0098] The embodiment of the present invention further provides a DC power transmission system converter grounding fault protection device, which is applied to a bipolar flexible DC power transmission system; comprising:

[0099] A fault collection module is configured to obtain a DC current of a high-voltage busbar of a converter, a DC current of a grounding busbar, and an AC three-phase current of a valve side of the converter;

[0100] A characteristic calculation module is configured to obtain a first characteristic value of a small current grounding fault of a converter according to a DC current of a high-voltage busbar of the converter and a DC current of a grounding electrode busbar, and to obtain a second characteristic value of a small current grounding fault of a converter according to an AC three-phase current on a valve side of the converter; and,

[0101] The fault judgment module is configured to judge whether a small current grounding fault of the converter occurs according to the first characteristic value and the second characteristic value of the small current grounding fault of the converter.

[0102] The characteristic calculation module obtains the first characteristic quantity of the converter low current grounding fault according to the converter high voltage bus DC current and the grounding electrode bus DC current, including:

[0103] Obtaining the DC current of the converter high voltage bus and the DC current of the grounding electrode bus;

[0104] The difference between the DC current of the high-voltage busbar of the converter and the DC current of the grounding electrode busbar is obtained, and the effective value of the difference is used as the first characteristic quantity of the low-current grounding fault of the converter.

[0105] The characteristic calculation module obtains the second characteristic value of the converter low current grounding fault according to the AC three-phase current on the converter valve side, including:

[0106] Obtaining the AC three-phase current on the converter valve side;

[0107] A zero-sequence current of the three-phase AC current on the valve side of the converter is obtained, and the zero-sequence current is used as a second characteristic quantity of a small current grounding fault of the converter.

[0108] The fault judgment module judges whether a small current grounding fault of the converter occurs according to the first characteristic value and the second characteristic value of the small current grounding fault of the converter, including:

[0109] When conditions one to four are met at the same time, it is determined that a small current grounding fault of the converter occurs;

[0110] Condition 1: The flexible DC transmission system is in bipolar operation mode;

[0111] Condition 2: the first characteristic value of the converter low-current grounding fault is greater than a set first threshold current;

[0112] Condition three: the second characteristic value of the converter low current grounding fault is greater than a set second threshold current;

[0113] Condition 4: The duration of conditions 1 to 3 being met is greater than the set delay, and no other protection actions are triggered.

[0114] Among them, in the condition 2, the first threshold current is greater than three times or more the maximum measurement error percentage of the measuring equipment in the flexible direct current transmission system in a steady state; in the condition 3, the second threshold current is greater than three times or more the maximum measurement error percentage of the measuring equipment in the flexible direct current transmission system in a steady state.

[0115] Among them, in the condition three, the second threshold current is less than the bridge arm inductor differential protection setting and the minimum triggering current of the bridge arm differential protection.

[0116] Among them, in the condition four, the set delay is simultaneously greater than the time setting values ​​of other protection actions in the flexible direct current transmission system.

[0117] Wherein, when the fault judgment module judges that a small current grounding fault of the converter occurs, it also includes:

[0118] After a pre-delay, the converter is locked and the AC incoming line circuit breaker connected to the converter is tripped, or the local pole power is reduced after a preset delay.

[0119] It also includes an alarm display module, which is configured to send out an alarm signal.

[0120] Accordingly, see Figure 3 , Figure 3The overall structure of the DC power transmission system converter grounding fault protection device of the embodiment of the present invention is illustrated. The embodiment of the present invention provides a DC power transmission system converter grounding fault protection device, which is applied to a bipolar flexible DC power transmission system. The device includes a fault acquisition module, a feature calculation module, a fault judgment module and an alarm display module, wherein the fault acquisition module is used to collect the converter high-voltage bus DC current IdH, the grounding bus DC current IdN and the valve side AC three-phase current Ivc, the feature calculation module is used to calculate the first feature quantity and the second feature quantity of the converter small current grounding fault, the fault judgment module is used to judge whether the converter small current grounding fault occurs; the alarm display module is used to send an alarm signal to facilitate the execution of protection actions and the arrangement of maintenance and fault clearing.

[0121] Accordingly, an embodiment of the present invention further provides an electronic device, see Figure 4 , Figure 4 The structure diagram of the electronic device of the embodiment of the present invention is illustrated. The electronic device includes a memory, a display, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, the steps of the above-mentioned DC power transmission system converter grounding fault protection method are implemented, and the display displays the fault alarm information and the area where the fault occurs. Since the DC power transmission system converter grounding fault protection method is described in detail above, it will not be repeated here.

[0122] In practical applications, the processor includes a field programmable gate array (FPGA), and the processor may be a central processing unit (CPU) or a digital signal processor (DSP). It is understandable that for different devices, the electronic device used to implement the function of the processor may be other, and the embodiment of the present invention does not specifically limit it.

[0123] The above-mentioned memory can be a volatile memory (volatile memory), such as a random access memory (RAM); or a non-volatile memory (non-volatile memory), such as a read-only memory (ROM), a flash memory, a hard disk (HDD) or a solid-state drive (SSD); or a combination of the above-mentioned types of memory, and provide instructions and data to the processor.

[0124] Accordingly, an embodiment of the present invention further provides a computer-readable storage medium on which a computer program is stored, and when the computer program is executed by a processor, the steps of the above-mentioned method for protecting a converter from a ground fault in a DC power transmission system are implemented. Since the above-mentioned method for determining the ground fault protection of a converter is described in detail, it will not be repeated here.

[0125] In the several embodiments provided by the present invention, it should be understood that the disclosed devices and methods can be implemented in other ways. The device embodiments described above are only schematic. For example, the division of the units is only a logical function division. There may be other division methods in actual implementation, such as: multiple units or components can be combined, or can be integrated into another system, or some features can be ignored or not executed. In addition, the coupling, direct coupling, or communication connection between the components shown or discussed can be through some interfaces, and the indirect coupling or communication connection of the devices or units can be electrical, mechanical or other forms.

[0126] It will be appreciated by those skilled in the art that embodiments of the present invention may be provided as methods, systems, or computer program products. Therefore, the present invention may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present invention may take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program codes. The schemes in the embodiments of the present invention may be implemented in various computer languages, for example, object-oriented programming language Java and literal scripting language JavaScript, etc.

[0127] The present invention is described with reference to flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to embodiments of the present invention. It should be understood that each process and / or block in the flowchart and / or block diagram, as well as the combination of processes and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0128] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0129] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process in the computer or other programmable device. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0130] Although the preferred embodiments of the present invention have been described, those skilled in the art may make other changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0131] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A method for protecting a converter from ground fault in a DC power transmission system, applied to a bipolar flexible DC power transmission system; characterized in that: include, Obtain a first characteristic value of a small current grounding fault of a converter and a second characteristic value of a small current grounding fault of a converter; wherein the first characteristic value of a small current grounding fault of a converter is obtained according to a DC current of a high-voltage bus of a converter and a DC current of a grounding bus, and the second characteristic value of a small current grounding fault of a converter is obtained according to an AC three-phase current on a valve side of the converter; According to the first characteristic value and the second characteristic value of the converter low current grounding fault, it is determined that a converter low current grounding fault occurs.

2. The method according to claim 1, characterized in that: The first characteristic quantity of the converter low current grounding fault is obtained according to the converter high voltage bus DC current and the grounding electrode bus DC current, and includes: Obtaining the DC current of the converter high voltage bus and the DC current of the grounding electrode bus; The difference between the DC current of the high-voltage busbar of the converter and the DC current of the grounding electrode busbar is obtained, and the effective value of the difference is used as the first characteristic quantity of the low-current grounding fault of the converter.

3. The method according to claim 1, characterized in that: The second characteristic quantity of the converter low current grounding fault is obtained according to the converter valve side AC three-phase current, including: Obtaining the AC three-phase current on the converter valve side; A zero-sequence current of the three-phase AC current on the valve side of the converter is obtained, and the zero-sequence current is used as a second characteristic quantity of a small current grounding fault of the converter.

4. The method according to claim 1, characterized in that: According to the first characteristic value and the second characteristic value of the converter small current grounding fault, judging that a converter small current grounding fault occurs, comprises: When conditions one to four are met at the same time, it is determined that a small current grounding fault of the converter occurs; Condition 1: The flexible DC transmission system is in bipolar operation mode; Condition 2: the first characteristic value of the converter low-current grounding fault is greater than a set first threshold current; Condition three: the second characteristic value of the converter low current grounding fault is greater than a set second threshold current; Condition 4: The duration of conditions 1 to 3 being met is greater than the set delay, and no other protection actions are triggered.

5. The method according to claim 4, characterized in that: In the second condition, the first threshold current is greater than three times or more the maximum measurement error percentage of the measuring device in the flexible DC transmission system in a steady state; in the third condition, the second threshold current is greater than three times or more the maximum measurement error percentage of the measuring device in the flexible DC transmission system in a steady state.

6. The method according to claim 4, characterized in that: In the condition three, the second threshold current is smaller than the bridge arm reactor differential protection setting value and the minimum trigger current of the bridge arm differential protection.

7. The method according to claim 4, characterized in that: In the fourth condition, the delay is set to be greater than the time setting values ​​of other protection actions in the flexible DC transmission system.

8. The method according to claim 1, characterized in that: When judging the occurrence of a small current grounding fault of the converter, it also includes: After a pre-delay, the converter is locked and the AC incoming line circuit breaker connected to the converter is tripped, or the local pole power is reduced after a preset delay.

9. The method according to claim 1, characterized in that: When judging the occurrence of a small current grounding fault of the converter, it also includes: Send out an alarm signal.

10. A DC power transmission system converter ground fault protection device, applied to a bipolar flexible DC power transmission system; characterized in that: include, A fault collection module is configured to obtain a DC current of a high-voltage busbar of a converter, a DC current of a grounding busbar, and an AC three-phase current of a valve side of the converter; A characteristic calculation module is configured to obtain a first characteristic value of a small current grounding fault of a converter according to a DC current of a high-voltage busbar of the converter and a DC current of a grounding electrode busbar, and to obtain a second characteristic value of a small current grounding fault of a converter according to an AC three-phase current on a valve side of the converter; as well as, The fault judgment module is configured to judge whether a small current grounding fault of the converter occurs according to the first characteristic value and the second characteristic value of the small current grounding fault of the converter.

11. The device according to claim 10, characterized in that: The characteristic calculation module obtains a first characteristic value of a converter low current grounding fault according to the converter high voltage bus DC current and the grounding electrode bus DC current. include, Obtaining the DC current of the converter high voltage bus and the DC current of the grounding electrode bus; The difference between the DC current of the high-voltage busbar of the converter and the DC current of the grounding electrode busbar is obtained, and the effective value of the difference is used as the first characteristic quantity of the low-current grounding fault of the converter.

12. The device according to claim 10, characterized in that: The characteristic calculation module obtains the second characteristic value of the converter low current grounding fault according to the AC three-phase current on the converter valve side. include, Obtaining the AC three-phase current on the converter valve side; A zero-sequence current of the three-phase AC current on the valve side of the converter is obtained, and the zero-sequence current is used as a second characteristic quantity of a small current grounding fault of the converter.

13. The device according to claim 10, characterized in that: The fault judgment module judges whether a small current grounding fault of the converter occurs according to the first characteristic value and the second characteristic value of the small current grounding fault of the converter, including: When conditions one to four are met at the same time, it is determined that a small current grounding fault of the converter occurs; Condition 1: The flexible DC transmission system is in bipolar operation mode; Condition 2: the first characteristic value of the converter low-current grounding fault is greater than a set first threshold current; Condition three: the second characteristic value of the converter low current grounding fault is greater than a set second threshold current; Condition 4: The duration of conditions 1 to 3 being met is greater than the set delay, and no other protection actions are triggered.

14. The device according to claim 13, characterized in that: In the second condition, the first threshold current is greater than three times or more the maximum measurement error percentage of the measuring device in the flexible DC transmission system in a steady state; in the third condition, the second threshold current is greater than three times or more the maximum measurement error percentage of the measuring device in the flexible DC transmission system in a steady state.

15. The device according to claim 13, characterized in that: In the condition three, the second threshold current is smaller than the bridge arm reactor differential protection setting value and the minimum trigger current of the bridge arm differential protection.

16. The device according to claim 13, characterized in that: In the fourth condition, the delay is set to be greater than the time setting values ​​of other protection actions in the flexible DC transmission system.

17. The device according to claim 11, characterized in that: When the fault judgment module judges that a small current grounding fault of the converter occurs, it also includes: After a pre-delay, the converter is locked and the AC incoming line circuit breaker connected to the converter is tripped, or the local pole power is reduced after a preset delay.

18. The device according to claim 11, characterized in that: It also includes an alarm display module configured to send out an alarm signal.

19. An electronic device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor; characterized in that: When the processor executes the computer program, the steps of the direct current transmission system converter ground fault protection method according to any one of claims 1 to 10 are implemented.

20. A computer-readable storage medium storing a computer program; characterized in that: When the computer program is executed by a processor, the steps of the method for protecting a converter from ground fault in a DC power transmission system as claimed in any one of claims 1 to 10 are implemented.

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