A method and system for detecting power grid faults based on current distribution characteristics in the module domain

Abstract

The present invention discloses a method and system for detecting power grid faults based on the current distribution characteristics of the modal domain, the method comprising: obtaining basic current data of the flexible direct current grid, the basic current data comprising the valve-side current of the pole line reactor and the branch current of the converter station; extracting the current fault component based on the basic current data to obtain the current fault component; forming a starting criterion based on the current fault component, and starting the criterion action; judging whether a current short circuit fault occurs, and when the starting criterion is met, using the fault identification criterion based on the two-end modal domain current distribution characteristics to perform fault detection. The present invention constructs two-end transient current direction information using the ratio of the valve-side current of the two-end pole line reactor to the bus current, has absolute selectivity for the fault line, and can provide backup protection for identifying the fault line, fault type and fault pole; the method of the present invention has high reliability and high identification accuracy.

Description

Technical Field

[0001] The present invention relates to the technical field of protection design of key equipment in a flexible direct current system, and in particular to a power grid fault detection method and system based on module domain current distribution characteristics. Background Art

[0002] Flexible DC grids often use overhead lines for long-distance power transmission. Affected by environmental, climatic and topographical factors, the probability of lightning strikes is significantly increased. Lightning strikes cause line insulation flashover and form a stable power frequency arc at a certain arcing rate, and the line then short-circuit faults. When faults such as single-pole grounding and bipolar short circuit occur in the protected line, the DC line detection starts and detects the fault quickly and reliably, so as to send a disconnection command to the DC circuit breaker for rapid isolation. However, due to the low damping and small inertia of the flexible DC grid, when a DC line short-circuit fault occurs, the DC voltage drops rapidly and the fault current rises rapidly within a few ms. This requires the protection scheme to have ms-level fault selectivity and reliability. In order to be able to quickly and accurately identify the faulty line within a few ms, it is necessary to propose a reliable and selective protection scheme.

[0003] According to the different sources of characteristic quantities, the protection schemes of flexible DC grid include single-end protection and longitudinal protection. Among them, single-end protection mainly uses single-end characteristic quantities to identify faulty lines. The single-end protection scheme of flexible DC grid mainly draws on the single-end protection scheme of DC lines in conventional DC transmission systems, that is, traveling wave protection and differential undervoltage protection are used as fast main protection, without communication and synchronization, and have advantages in speed, and are mainly used as main protection schemes. Longitudinal protection mainly uses double-end characteristic quantities to identify faulty lines. The longitudinal protection of flexible DC grid mainly draws on the longitudinal protection scheme of DC lines in conventional DC transmission systems, that is, double-end current changes are used as characteristic quantities, and communication and synchronization are required at both ends. It has advantages in selectivity and reliability, and is mainly used as a backup protection scheme.

[0004] The existing flexible DC grid protection schemes have the following shortcomings: (1) The single-ended protection scheme of the flexible DC grid is overly dependent on the amplitude of the electrical quantity and its rate of change, is extremely sensitive to the transition resistance, and has low response sensitivity to high transition resistance fault conditions; (2) The longitudinal protection scheme of the flexible DC grid is overly dependent on the current change, is extremely susceptible to noise interference, and is prone to protection misoperation when the interference is large. Summary of the invention

[0005] The technical problem to be solved by the present invention is that in the existing flexible DC grid fault detection method, (1) the single-end protection scheme of the flexible DC grid is overly dependent on the electrical quantity amplitude and its change rate, is extremely sensitive to the transition resistance, and has low response sensitivity to the high transition resistance fault condition; (2) the longitudinal protection scheme of the flexible DC grid is overly dependent on the current change, is extremely susceptible to noise interference, and is prone to protection malfunction when the interference is large; thus, the existing flexible DC grid fault detection method has the problems of low reliability and low accuracy.

[0006] The purpose of the present invention is to provide a power grid fault detection method and system based on the mode domain current distribution characteristics, which constructs two-terminal transient current direction information by using the ratio of the valve side current of the two-terminal pole line reactor to the bus current. It has absolute selectivity for the fault line and can provide backup protection for identifying the fault line, fault type and fault pole. The method has high reliability and high identification accuracy.

[0007] The present invention is achieved through the following technical solutions:

[0008] In a first aspect, the present invention provides a method for detecting power grid faults based on mode domain current distribution characteristics, the method comprising:

[0009] Acquiring basic current data of the flexible DC grid, wherein the basic current data includes valve-side current of a pole line reactor and branch current of a converter station;

[0010] Based on the basic current data, current fault components are extracted to obtain current fault components;

[0011] A starting criterion is formed based on the current fault component, and the criterion action is started; it is determined whether a current short circuit fault occurs, and when the starting criterion is met, a fault identification criterion based on the double-end mode domain current distribution characteristic is used to perform fault detection.

[0012] The working principle is: based on the existing flexible DC grid fault detection method, (1) the flexible DC grid single-end protection scheme is overly dependent on the electrical quantity amplitude and its change rate, is extremely sensitive to transition resistance, and has low response sensitivity to high transition resistance fault conditions; (2) the flexible DC grid longitudinal protection scheme is overly dependent on the current change, is easily affected by noise interference, and is prone to protection misoperation when the interference is large; which leads to the existing flexible DC grid fault detection method has low reliability and low accuracy. The present invention adopts a fault identification criterion based on the double-end mode domain current distribution characteristic for fault detection. First, the starting criterion uses the DC line current change as its characteristic quantity to detect short-circuit faults in the DC grid, which is the starting element of the subsequent fault identification criterion; secondly, the identification criterion uses the double-end mode domain current distribution characteristic as its characteristic quantity. When the protection starting criterion meets the requirements, the longitudinal protection scheme will perform fault identification, including fault line identification, fault type identification and fault pole identification.

[0013] The present invention constructs double-terminal transient current direction information by utilizing the ratio of valve-side current of a double-terminal pole line reactor to bus current, has absolute selectivity for fault lines, and can provide backup protection for identifying fault lines, fault types and fault poles.

[0014] Furthermore, the startup criterion includes the change in the fault current on the valve side of the pole line reactor and the protection startup threshold current. The startup criterion is as follows:

[0015]

[0016] In the formula, || is the absolute value operator, i vp (t),i vm (t),i vn (t) are the time domain fault current components of the positive line, metal return line, and negative line on the valve side of the pole line reactor, respectively. They are obtained by subtracting the current before the fault from the current after the fault. In addition, I Thre1 It is the protection start threshold current.

[0017] Furthermore, the protection start threshold current I Thre1 is selected to be smaller than the minimum current change on the line side of the current limiting reactor when a high-resistance fault occurs at the end of the line.

[0018] Furthermore, the fault detection using a fault identification criterion based on the double-terminal mode domain current distribution characteristic includes:

[0019] The fault line identification criterion based on the double-terminal mode domain current distribution characteristics is used to identify the fault line;

[0020] The fault type identification criterion based on the double-terminal mode domain current distribution characteristics is used to identify the fault type;

[0021] The fault pole identification criterion based on the double-terminal mode domain current distribution characteristics is used for fault pole identification.

[0022] Furthermore, the fault line identification uses the ratio of the fault current on the valve side of the double-terminal pole line reactor to the amplitude of the 2-mode component of the bus current to obtain the double-terminal transient current direction information, so as to realize accurate identification of the fault line; the fault line identification criterion is:

[0023] D1=1&D2=1 (1.2)

[0024] In the formula, D1 and D2 are the transient current direction information of terminal 1 and terminal 2 respectively;

[0025] When the fault line identification criterion formula (1.2) meets the requirements, it is judged that an in-zone fault has occurred; otherwise, it is an out-of-zone fault.

[0026] Further, D1 is determined by the first transient current ratio K1 of terminal 1, and D2 is determined by the second transient current ratio K2 of terminal 2; specifically:

[0027] When K1>1, the transient current direction information D1=1 at terminal 1, otherwise, D1=0;

[0028] When K2>1, the transient current direction information D2=1 at the 2nd terminal, otherwise, D2=0;

[0029] The expressions of the first transient current ratio K1 and the second transient current ratio K2 are as follows:

[0030]

[0031] In the formula, i f12 (t) is the 2nd mode component of the fault current on the valve side of the 1-terminal pole line reactor, i MMC12 (t) is the 2nd mode component of the fault current in the branch of the converter station at end 1, i f22 (t) is the 2-mode component of the fault current on the valve side of the 2-terminal line reactor, i MMC22 (t) is the 2-mode component of the fault current in the branch of the two-terminal converter station, and N is the number of sampling points in a preset time window (for example, 1 ms).

[0032] Furthermore, the fault current 2 mode component i f12 (t), i MMC12 (t), i MMC12 (t), i MMC22 (t) The time domain fault current component is transformed into the mode domain quantity after phase mode transformation:

[0033]

[0034] In the formula, i vp1 (t),i vm1(t),i vn1 (t) are the time domain fault current components of the positive line, metal return line, and negative line on the valve side of the 1-terminal line reactor, respectively. pMMC1 (t),i mMMC1 (t),i nMMC1 (t) are the time domain fault current components of the positive line, metal return line and negative line of the converter station at the 1st end, respectively, i vp2 (t),i vm2 (t),i vn2 (t) are the time domain fault current components of the positive line, metal return line, and negative line on the valve side of the two-terminal line reactor, respectively. pMMC2 (t),i mMMC2 (t),i nMMC2 (t) are the time domain fault current components of the positive line, metal return line and negative line of the two-end converter station respectively.

[0035] Furthermore, the fault type identification criterion is composed of the amplitude and threshold of the 0, 1, and 2 mode fault currents, and the fault type identification criterion is:

[0036]

[0037] In the formula, Fault current zero mode component i f10 (t), i f20 (t), fault current 1 mode component i f11 (t), i f21 (t), fault current 2 mode component i f12 (t), i f22 (t) can be calculated by formula (1.4); T s is the time interval between two sampling points; T w is the time window. To satisfy the reliability of the criterion calculation, the time window T w Take 2ms; t1 is the protection start time, K rel1 is the threshold for fault type identification. In this paper, K rel1 =0.01.

[0038] When the fault type identification criterion formula (1.5) meets the requirements, the fault type is a bipolar short circuit fault; otherwise, the fault type is a unipolar short circuit fault.

[0039] Furthermore, the fault pole identification criterion is:

[0040]

[0041] When the fault pole identification criterion formula (1.6) meets the requirements, the fault pole is judged to be the negative pole; otherwise, the fault pole is the positive pole.

[0042] In a second aspect, the present invention further provides a power grid fault detection system based on the mode domain current distribution characteristics, the system supports the power grid fault detection method based on the mode domain current distribution characteristics; the system comprises:

[0043] An acquisition unit, used for acquiring basic current data of the flexible DC grid, wherein the basic current data includes valve-side current of a pole line reactor and branch current of a converter station;

[0044] A current fault component extraction unit, configured to extract the current fault component based on the basic current data to obtain the current fault component;

[0045] A starting criterion unit, used to form a starting criterion based on the current fault component and start a criterion action;

[0046] The fault detection unit is used to determine whether a current short circuit fault occurs according to a starting criterion; when the starting criterion is met, a fault identification criterion based on the double-end mode domain current distribution characteristic is used to perform fault detection.

[0047] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0048] The present invention provides a method and system for detecting power grid faults based on the mode domain current distribution characteristics. The method uses a fault identification criterion based on the double-end mode domain current distribution characteristics to perform fault detection. First, the starting criterion uses the DC line current change as its characteristic quantity to detect short-circuit faults in the DC power grid, which is the starting element of the subsequent fault identification criterion. Secondly, the identification criterion uses the double-end mode domain current distribution characteristics as its characteristic quantity. When the protection starting criterion meets the requirements, the longitudinal protection scheme will perform fault identification, including fault line identification, fault type identification and fault pole identification. The present invention uses the ratio of the valve side current of the double-end pole line reactor to the bus current to construct double-end transient current direction information, has absolute selectivity for the fault line, and can provide backup protection for identifying the fault line, fault type and fault pole. The method of the present invention has high reliability and high identification accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention. In the drawings:

[0050] Figure 1 The present invention is a flow chart of a power grid fault detection method based on mode domain current distribution characteristics.

[0051] Figure 2 The present invention is a detailed flow chart of a power grid fault detection method based on mode domain current distribution characteristics.

[0052] Figure 3The present invention is a schematic diagram of the structure of a power grid fault detection system based on mode domain current distribution characteristics. DETAILED DESCRIPTION

[0053] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments and drawings. The exemplary embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0054] Based on the existing flexible DC grid fault detection methods, (1) the single-ended protection scheme of the flexible DC grid is overly dependent on the amplitude of the electrical quantity and its rate of change, and is extremely sensitive to the transition resistance, and has low sensitivity to the high transition resistance fault condition; (2) the longitudinal protection scheme of the flexible DC grid is overly dependent on the current change, and is extremely susceptible to noise interference, and is prone to protection misoperation when the interference is large; This leads to the problems of low reliability and low accuracy of the existing flexible DC grid fault detection methods.

[0055] The present invention designs a power grid fault detection method based on the mode domain current distribution characteristics. Different from the single-ended mode of the prior art, the present invention adopts a fault identification criterion based on the double-ended mode domain current distribution characteristics for fault detection. First, the starting criterion adopts the DC line current change as its characteristic quantity to detect the short-circuit fault in the DC power grid, which is the starting element of the subsequent fault identification criterion; secondly, the identification criterion adopts the double-ended mode domain current distribution characteristics as its characteristic quantity. When the protection starting criterion meets the requirements, the longitudinal protection scheme will perform fault identification, including fault line identification, fault type identification and fault pole identification. The present invention uses the ratio of the valve side current of the double-ended pole line reactor to the bus current to construct the double-ended transient current direction information, which has absolute selectivity for the fault line and can provide backup protection for identifying the fault line, fault type and fault pole; the method of the present invention has high reliability and high identification accuracy.

[0056] Example 1

[0057] like Figure 1 and Figure 2 As shown, the present invention provides a power grid fault detection method based on mode domain current distribution characteristics, the method comprising:

[0058] Step 1, obtaining basic current data of the flexible DC grid, wherein the basic current data includes valve-side current of a pole line reactor and branch current of a converter station;

[0059] Step 2, extracting current fault components based on the basic current data to obtain current fault components;

[0060] Step 3, forming a starting criterion based on the current fault component, and starting the criterion action;

[0061] The startup criteria include the fault current variation of the pole line reactor valve side and the protection startup threshold current. The startup criteria are as follows:

[0062]

[0063] In the formula, || is the absolute value operator, i vp (t),i vm (t),i vn (t) are the time domain fault current components of the positive line, metal return line, and negative line on the valve side of the pole line reactor, respectively. They are obtained by subtracting the current before the fault from the current after the fault. In addition, I Thre1 It is the protection start threshold current.

[0064] Specifically, the protection start threshold current I Thre1 The selection of needs to be smaller than the minimum current change on the line side of the current limiting reactor when a high-resistance fault occurs at the end of the line.

[0065] Step 4, determine whether a current short circuit fault occurs, and if the start criterion is met, use a fault identification criterion based on the double-end mode domain current distribution characteristic to perform fault detection. Wherein, using a fault identification criterion based on the double-end mode domain current distribution characteristic to perform fault detection includes:

[0066] The fault line identification criterion based on the double-terminal mode domain current distribution characteristics is used to identify the fault line;

[0067] The fault type identification criterion based on the double-terminal mode domain current distribution characteristics is used to identify the fault type;

[0068] The fault pole identification criterion based on the double-terminal mode domain current distribution characteristics is used for fault pole identification.

[0069] (i) Fault line identification: The fault line identification uses the ratio of the fault current on the valve side of the double-terminal pole line reactor to the amplitude of the 2-mode component of the bus current to obtain the double-terminal transient current direction information, so as to accurately identify the fault line; the fault line identification criterion is:

[0070] D1=1&D2=1 (1.2)

[0071] In the formula, D1 and D2 are the transient current direction information of terminal 1 and terminal 2 respectively;

[0072] When the fault line identification criterion formula (1.2) meets the requirements, it is judged that an in-zone fault has occurred; otherwise, it is an out-of-zone fault.

[0073] As a further implementation, D1 is determined by the first transient current ratio K1 of terminal 1, and D2 is determined by the second transient current ratio K2 of terminal 2; specifically:

[0074] When K1>1, the transient current direction information D1=1 at terminal 1, otherwise, D1=0;

[0075] When K2>1, the transient current direction information D2=1 at the 2nd terminal, otherwise, D2=0;

[0076] The expressions of the first transient current ratio K1 and the second transient current ratio K2 are as follows:

[0077]

[0078] In the formula, i f12 (t) is the 2nd mode component of the fault current on the valve side of the 1-terminal pole line reactor, i MMC12 (t) is the 2nd mode component of the fault current in the branch of the converter station at end 1, i f22 (t) is the 2-mode component of the fault current on the valve side of the 2-terminal line reactor, i MMC22 (t) is the 2-mode component of the fault current in the branch of the two-terminal converter station, and N is the number of sampling points in a preset time window (for example, 1 ms).

[0079] Among them, the fault current 2 mode component i f12 (t), i MMC12 (t), i MMC12 (t), i MMC22 (t) The time domain fault current component is transformed into the mode domain quantity after phase mode transformation:

[0080]

[0081] In the formula, i vp1 (t),i vm1 (t),i vn1 (t) are the time domain fault current components of the positive line, metal return line, and negative line on the valve side of the 1-terminal line reactor, respectively. pMMC1 (t),i mMMC1 (t),i nMMC1 (t) are the time domain fault current components of the positive line, metal return line and negative line of the converter station at the 1st end, respectively, i vp2 (t),i vm2 (t),i vn2 (t) are the time domain fault current components of the positive line, metal return line, and negative line on the valve side of the two-terminal line reactor, respectively. pMMC2 (t),i mMMC2 (t),i nMMC2 (t) are the time domain fault current components of the positive line, metal return line and negative line of the two-end converter station respectively.

[0082] (ii) Fault type identification: The fault type identification criteria are composed of the amplitude and threshold of the 0, 1, and 2 mode fault currents. The fault type identification criteria are:

[0083]

[0084] In the formula, Fault current zero mode component i f10 (t), i f20 (t), fault current 1 mode component i f11 (t), i f21 (t), fault current 2 mode component i f12 (t), i f22 (t) can be calculated by formula (1.4); T s is the time interval between two sampling points; T w is the time window. To satisfy the reliability of the criterion calculation, the time window T w Take 2ms; t1 is the protection start time, K rel1 is the threshold for fault type identification. In this paper, K rel1 =0.01.

[0085] When the fault type identification criterion formula (1.5) meets the requirements, the fault type is a bipolar short circuit fault; otherwise, the fault type is a unipolar short circuit fault.

[0086] (iii) Fault pole identification: The fault pole identification criterion is:

[0087]

[0088] When the fault pole identification criterion formula (1.6) meets the requirements, the fault pole is judged to be the negative pole; otherwise, the fault pole is the positive pole.

[0089] Example 2

[0090] like Figure 3 As shown, the difference between this embodiment and embodiment 1 is that this embodiment further provides a power grid fault detection system based on the mode domain current distribution characteristics, and the system supports the power grid fault detection method based on the mode domain current distribution characteristics described in embodiment 1; the system includes:

[0091] An acquisition unit, used for acquiring basic current data of the flexible DC grid, wherein the basic current data includes valve-side current of a pole line reactor and branch current of a converter station;

[0092] A current fault component extraction unit, configured to extract the current fault component based on the basic current data to obtain the current fault component;

[0093] A starting criterion unit, used to form a starting criterion based on the current fault component and start a criterion action;

[0094] The fault detection unit is used to determine whether a current short circuit fault occurs according to a starting criterion; when the starting criterion is met, a fault identification criterion based on the double-end mode domain current distribution characteristic is used to perform fault detection.

[0095] The execution process of each unit can be performed according to the process steps of a power grid fault detection method based on module-domain current distribution characteristics described in Example 1, and will not be described one by one in this embodiment.

[0096] Those skilled in the art will appreciate that the embodiments of the present application may be provided as methods, systems, or computer program products. Therefore, the present application may adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment in combination with software and hardware. Moreover, the present application may adopt the form of a computer program product implemented in one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) that include computer-usable program code.

[0097] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes 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 generate 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.

[0098] 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 including 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.

[0099] 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. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0100] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A power grid fault detection method based on mode domain current distribution characteristics, characterized in that: The method includes: Acquiring basic current data of the flexible DC grid, wherein the basic current data includes valve-side current of a pole line reactor and branch current of a converter station; Based on the basic current data, current fault components are extracted to obtain current fault components; Based on the current fault component, a starting criterion is formed and the criterion action is started; whether a current short circuit fault occurs is determined, and when the starting criterion is met, a fault identification criterion based on the double-end mode domain current distribution characteristic is used to perform fault detection; The fault detection is performed by using a fault identification criterion based on the double-terminal mode domain current distribution characteristic, including: The fault line identification criterion based on the double-terminal mode domain current distribution characteristics is used to identify the fault line; The fault type identification criterion based on the double-terminal mode domain current distribution characteristics is used to identify the fault type; The fault pole identification criterion based on the double-terminal mode domain current distribution characteristics is used to identify the fault pole; The fault line identification is to obtain the two-terminal transient current direction information by using the ratio of the fault current on the valve side of the two-terminal pole line reactor to the amplitude of the 2-mode component of the bus current, so as to realize the fault line identification; the fault line identification criterion is: D1=1&D2=1 In the formula, D1 and D2 are the transient current direction information of terminal 1 and terminal 2 respectively; When the fault line identification criteria meet the requirements, it is judged that an internal fault has occurred; otherwise, it is an external fault; D1 is determined by the first transient current ratio K1 at terminal 1, and D2 is determined by the second transient current ratio K2 at terminal 2; specifically: When K1>1, the transient current direction information D1=1 at terminal 1, otherwise, D1=0; When K2>1, the transient current direction information D2=1 at the 2nd terminal, otherwise, D2=0; The expressions of the first transient current ratio K1 and the second transient current ratio K2 are as follows: In the formula, i f12 (t) is the 2nd mode component of the fault current on the valve side of the 1-terminal pole line reactor, i MMC12 (t) is the 2nd mode component of the fault current in the branch of the converter station at end 1, i f22 (t) is the 2-mode component of the fault current on the valve side of the 2-terminal line reactor, i MMC22 (t) is the 2-mode component of the fault current in the branch of the 2-terminal converter station, and N is the number of sampling points in the preset time window.

2. A power grid fault detection method based on mode domain current distribution characteristics according to claim 1, characterized in that: The startup criteria include the fault current variation of the pole line reactor valve side and the protection startup threshold current. The startup criteria are as follows: In the formula, || is the absolute value operator, i vp (t),i vm (t),i vn (t) are the time domain fault current components of the positive line, metal return line, and negative line on the valve side of the pole line reactor, respectively. They are obtained by subtracting the current before the fault from the current after the fault; I Thre1 It is the protection start threshold current.

3. A method for detecting power grid faults based on mode domain current distribution characteristics according to claim 2, characterized in that: Protection start threshold current I Thre1 is selected to be smaller than the minimum current change on the line side of the current limiting reactor when a high-resistance fault occurs at the end of the line.

4. A method for detecting power grid faults based on mode domain current distribution characteristics according to claim 1, characterized in that: The fault current 2 mode component i f12 (t), i MMC12 (t), i MMC12 (t), i MMC22 (t) The time domain fault current component is transformed into the mode domain quantity after phase mode transformation: In the formula, i vp1 (t),i vm1 (t),i vn1 (t) are the time domain fault current components of the positive line, metal return line, and negative line on the valve side of the 1-terminal line reactor, respectively. pMMC1 (t),i mMMC1 (t),i nMMC1 (t) are the time domain fault current components of the positive line, metal return line and negative line of the converter station at the 1st end, respectively, i vp2 (t),i vm2 (t),i vn2 (t) are the time domain fault current components of the positive line, metal return line, and negative line on the valve side of the two-terminal line reactor, respectively. pMMC2 (t),i mMMC2 (t),i nMMC2 (t) are the time domain fault current components of the positive line, metal return line and negative line of the two-end converter station respectively.

5. The method for detecting power grid faults based on mode domain current distribution characteristics according to claim 1, characterized in that: The fault type identification criterion is: In the formula, T s is the time interval between two sampling points; T w is the time window. To satisfy the reliability of the criterion calculation, the time window T w Take 2ms; t1 is the protection start time, K rel1 is the threshold for fault type identification; When the fault type identification criterion meets the requirements, the fault type is a bipolar short circuit fault; otherwise, the fault type is a unipolar short circuit fault.

6. The method for detecting power grid faults based on mode domain current distribution characteristics according to claim 1, characterized in that: The fault pole identification criterion is: When the fault pole identification criterion meets the requirements, the fault pole is judged to be the negative pole; otherwise, the fault pole is the positive pole.

7. A power grid fault detection system based on mode domain current distribution characteristics, characterized in that: The system supports a power grid fault detection method based on mode domain current distribution characteristics as described in any one of claims 1 to 6; the system comprises: An acquisition unit, used for acquiring basic current data of the flexible DC grid, wherein the basic current data includes valve-side current of a pole line reactor and branch current of a converter station; A current fault component extraction unit, configured to extract the current fault component based on the basic current data to obtain the current fault component; A starting criterion unit, used to form a starting criterion based on the current fault component and start a criterion action; The fault detection unit is used to determine whether a current short circuit fault occurs according to a starting criterion; when the starting criterion is met, a fault identification criterion based on the double-end mode domain current distribution characteristic is used to perform fault detection.

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