Flexible DC power grid line fault pilot protection method

Abstract

The invention discloses a flexible direct-current power grid line fault pilot protection method, which consists of three parts, namely a fault starting criterion, a fault direction criterion and a pilot protection criterion, is suitable for net-shaped flexible direct-current power grid line fault protection, and can be used as a backup protection method matched with a single-ended ultra-fast main protection method. The fault starting criterion detects an abnormal state through a change rate of a pole current, the fault direction criterion judges fault direction information through an integral size of a line mode fault current component, and the pilot protection criterion judges whether an internal line fault exists or not by judging fault directions and a time difference of two ends of a line. According to the method, only current signals need to be measured, deployment is convenient, fault characteristic quantity difference caused by the fault direction is enlarged by means of the integral quantity, the protection boundary is clear, high-resistance faults can be reliably recognized, the anti-interference capability is high, and the actual application prospect is met.

Description

Technical Field

[0001] This invention belongs to the field of DC power grid line fault protection, and specifically relates to a longitudinal protection method for flexible DC power grid line faults. Background Technology

[0002] With the rapid development of renewable energy sources such as wind and solar power, future power systems will be dominated by new energy sources. However, the large fluctuations, long durations, and seasonality of wind and solar power output lead to simultaneous power shortages and curtailment issues, posing significant challenges to balancing power supply in both time and space. Flexible DC systems based on voltage source converters (VSCs) enable high-penetration, large-scale, and smooth grid integration of renewable energy, achieving flexible grid interconnection. This is considered a major technological revolution in the future development of power systems. Modular multilevel converter (MMC) technology has driven the development of flexible DC systems, improving voltage levels and reducing operating losses.

[0003] The first step in line protection is to construct fault characteristic quantities that can reflect fault information. Based on these fault characteristic quantities, it is determined whether a fault has occurred, the area where the fault occurred, and the type of fault. Based on the location used to construct the characteristic quantity signal, methods can be divided into single-ended and double-ended methods. After a fault occurs in a flexible DC system, it can be considered as an equivalent capacitor discharging to ground through equivalent reactance and resistance. The fault loop damping is small, the DC fault current rises rapidly and has no zero-crossing point. Due to the weak overcurrent capability of MMCs based on turn-off devices, and the immaturity of high-performance DC circuit breakers (DCCBs), it is generally believed that the main protection of flexible DC power grids must be single-ended transient quantities. However, existing single-ended fault protection methods based on the properties of line boundary elements (current-limiting reactors) have limitations such as difficulty in coordinating the four protection characteristics, complex setting thresholds, and low sensitivity to high-resistance faults. Summary of the Invention

[0004] Purpose of the invention: The purpose of this invention is to provide a longitudinal protection method for flexible DC power grid line faults, which can overcome the problem that traditional single-end ultra-fast main protection for flexible DC power grids is difficult to identify high-resistance line faults.

[0005] Technical solution: The present invention provides a longitudinal fault protection method for flexible DC power grid lines, comprising:

[0006] Triggering criterion: Real-time measurement of the positive line current of the line where signal measurement point R12 is located in the flexible DC power grid. and negative line current According to the positive line current The rate of change of the positive line current was calculated. According to the negative line current The rate of change of the negative electrode current was calculated. The rate of change of the positive line current Rate of change of negative line current The system compares the data with a preset threshold and determines whether an anomaly has occurred in the flexible DC power grid line. If an anomaly occurs, the system proceeds to the next step and initiates the direction criterion.

[0007] Direction criterion: Record the sampling point time that satisfies the activation criterion. The sampling order number is The positive line current is calculated using the steady-state current of the DC line. and negative line current The change in quantity; using the linear model decoupling matrix For positive line current and negative line current The changes are decoupled to obtain the line-mode fault current component and the zero-mode fault current component of the fault line; the line-mode fault current integral is calculated based on the line-mode fault current component of the fault line; the line-mode fault current integral is compared with two preset threshold coefficients, and the fault direction of the flexible DC grid line is determined based on the comparison result; after determining the fault direction, the next step is to activate the longitudinal protection criterion.

[0008] Longitudinal protection criterion: After determining whether the fault direction is forward or reverse, the protection device at the local signal measurement point sends the sampling point time through the communication system. The fault direction information is sent to the other end of the line; simultaneously, the protection device at the local signal measurement point receives the sampling point time from the protection device at the other end of the line. And fault direction information; when both protection devices at both ends of the line determine it as a positive fault, and the sampling point time is... and sampling point time If the judgment condition is met, the line is judged to be a faulty line; otherwise, it is a non-faulty line.

[0009] Furthermore, the rate of change of the positive line current... Rate of change of negative line current The system compares the data with a preset threshold and determines whether an anomaly has occurred in the flexible DC power grid line based on the comparison result, including:

[0010] ;

[0011] in, The threshold value for setting the start-up criterion; if the rate of change of the positive line current... Rate of change of negative line current If the above formula is satisfied, it is determined that the flexible DC power grid line has an anomaly, and the criterion direction criterion is activated.

[0012] Furthermore, the positive line current is calculated using the steady-state current of the DC line. and negative line current The change in is expressed as follows:

[0013] ;

[0014] ;

[0015] in, This represents the change in the positive line current. This represents the change in current in the negative electrode line. This represents the steady-state current of a DC line.

[0016] Furthermore, the use of the line-mode decoupling matrix For positive line current and negative line current By decoupling the changes in the quantity, the line-mode fault current component and the zero-mode fault current component of the fault line are obtained, and the calculation formula is as follows:

[0017] ;

[0018] ;

[0019] in, The fault current component is the line-mode fault current component of the faulty line. This is the zero-mode fault current component; This represents the change in the positive line current. This represents the change in current in the negative electrode line.

[0020] Furthermore, the integral of the line-mode fault current is calculated based on the line-mode fault current component of the faulty line, using the following formula:

[0021] ;

[0022] in, The integral of the line-mode fault current; The sampling sequence number is required to meet the activation criterion; The total number of sampling points for integration.

[0023] Furthermore, the step of comparing the integral of the line-mode fault current with two preset threshold coefficients and determining the fault direction of the flexible DC power grid line based on the comparison result is as follows:

[0024] ;

[0025] in, and All are threshold coefficients, and all are greater than 0.

[0026] Furthermore, the threshold coefficient and The value is determined based on the noise intensity and the total number of sampling points used for integration. Decide.

[0027] Furthermore, when both protection devices at both ends of the line determine a positive fault, and the sampling point time... and sampling point time If the judgment conditions are met, the line is determined to be a faulty line; otherwise, it is a non-faulty line. The judgment conditions are as follows:

[0028] ;

[0029] in, The total length of the faulty line; The estimated speed of the traveling wave propagation in the linear mode fault; For reliability coefficient, .

[0030] Furthermore, this protection method is applicable to both single-pole grounding faults and double-pole short-circuit faults.

[0031] Furthermore, the communication system is a private wireless network, a public wireless network, or a private fiber optic network.

[0032] Beneficial effects: Compared with the prior art, the significant technical effects of the present invention are as follows: (1) The method of the present invention only uses the measured fault current signal, without the need for the fault voltage signal, which is convenient for deployment and implementation, has low cost, and has high reliability; (2) The method of the present invention uses the integral of the line-mode fault current to determine the fault direction. For the fault in the positive direction of the same DC bus, its value is positive, and for the fault in the opposite direction, its value is negative. The protection boundary is clear and can reliably identify high-resistance faults; (3) The method of the present invention uses the integral quantity to expand the difference in fault characteristic quantity caused by the fault direction, which has strong anti-interference ability and can be used as a backup protection method in conjunction with the single-ended quantity ultra-fast main protection, which meets the practical application prospects. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the process of the present invention;

[0034] Figure 2 This is a schematic diagram of the additional fault states after a fault in a flexible DC power grid line in this invention.

[0035] Figure 3 This is a schematic diagram of the Peterson equivalent circuit after a line fault in this invention.

[0036] Figure 4 This is a schematic diagram of the fault protection results in this invention. Detailed Implementation

[0037] The technical solution of the present invention will now be described in detail with reference to specific embodiments and accompanying drawings.

[0038] This invention provides a longitudinal fault protection method for flexible DC power grid lines, applicable to various types of DC lines, including overhead lines and cable lines. It is suitable for both single-pole grounding faults and bipolar short-circuit faults, and applicable to flexible DC transmission networks of various voltage levels and topologies. It can be used as backup protection in conjunction with single-ended ultra-fast main protection methods. This invention is applicable to flexible DC power grids employing pseudo-bipolar or true bipolar connections, and is suitable for DC bus connections with... ( A mesh system with 10 DC lines, in this embodiment, the DC bus is connected to 10 DC lines. Take a DC line as an example.

[0039] like Figure 1 As shown, the flexible DC power grid line fault longitudinal protection method of the present invention specifically includes the following steps:

[0040] S1. Startup Criterion: Real-time measurement of the positive line current of the line where signal measurement point R12 is located in the flexible DC power grid. and negative line current According to the positive line current The rate of change of the positive line current was calculated. According to the negative line current The rate of change of the negative electrode current was calculated. The rate of change of the positive line current Rate of change of negative line current The system compares the data with a preset threshold and determines whether an anomaly has occurred in the flexible DC power grid line. If an anomaly occurs, the system proceeds to the next step and initiates the direction criterion.

[0041] Among them, the rate of change of the positive line current Rate of change of negative line current The system compares the data with a preset threshold and determines whether an anomaly has occurred in the flexible DC power grid line based on the comparison result, including:

[0042] ;

[0043] in, The threshold value for setting the trigger criterion should be able to detect... The abnormal state caused by a single-pole grounding fault. After a single-pole grounding fault occurs, the location of the fault point can be calculated. The size is:

[0044] ;

[0045] in, For zero-mode impedance, The power transmission lines Typically 400Ω, It is generally around 250Ω, therefore when hour, . The size is related to the sampling rate, but should not exceed 46kV.

[0046] If the rate of change of the positive electrode current Rate of change of negative line current If the above formula is satisfied, the flexible DC power grid line is considered to have an anomaly, and the direction criterion is activated. After the activation criterion is satisfied, subsequent protection is triggered; otherwise, sampling and detection continue.

[0047] According to the superposition theorem, the additional fault state of a flexible DC power grid after a fault occurs in a DC line is as follows: Figure 2 As shown, in this embodiment, DC bus 1 is connected to... Taking a DC line as an example, signal measurement points R12 and R21 are protection measurement points within the faulty line area, R12 is a measurement point for non-faulty lines on the same DC bus 1, and R41 is a measurement point at the far end of non-faulty lines on the same DC bus 1. The surge impedance of the DC line can be approximated as resistive; L represents the size of the current-limiting reactor in the DC line; and DCCB is a high-efficiency, high-capacity DC circuit breaker, such as a hybrid DC circuit breaker or a mechanically vibrating DC circuit breaker. Given the equivalent impedance of the MMC before the submodule latches up, we have , , , For the size of the MMC bridge arm reactor, The size of the capacitor in the MMC submodule. The number of bridge arm sub-modules. It can be ignored. The transition resistance at the fault point. This is the line's rated voltage. It is a step function. The magnitude of the fault voltage generated at the fault point. The magnitude of the fault current at the faulty line port. This refers to the magnitude of the fault current at the non-faulty line port of the same DC bus. The magnitude of the fault current flowing from the MMC to DC bus 1.

[0048] S2, Direction Criterion: Record the sampling point time that satisfies the activation criterion. The sampling order number is The positive line current is calculated using the steady-state current of the DC line. and negative line current The change in quantity; using the linear model decoupling matrix For the positive line current I P (t) and negative line current I N The change in (t) is decoupled to obtain the line-mode fault current component and the zero-mode fault current component of the fault line; the line-mode fault current integral is calculated based on the line-mode fault current component of the fault line; the line-mode fault current integral is compared with two preset threshold coefficients, and the fault direction of the flexible DC grid line is determined based on the comparison result; after determining the fault direction, the next step is to activate the longitudinal protection criterion.

[0049] In this embodiment, the positive line current is calculated using the steady-state current of the DC line. and negative line current The change in is expressed as follows:

[0050] ;

[0051] ;

[0052] in, This represents the change in the positive line current. This represents the change in current in the negative electrode line. This represents the steady-state current of a DC line.

[0053] In this embodiment, a line-mode decoupling matrix is ​​used. For positive line current and negative line current By decoupling the changes in the quantity, the line-mode fault current component and the zero-mode fault current component of the fault line are obtained, and the calculation formula is as follows:

[0054] ;

[0055] ;

[0056] in, The fault current component is the line-mode fault current component of the faulty line. This is the zero-mode fault current component; This represents the change in the positive line current. This represents the change in current in the negative electrode line.

[0057] In this embodiment, the line-mode fault current integral is calculated based on the line-mode fault current component of the fault line, and can be expressed in discrete form as follows:

[0058] ;

[0059] in, The integral of the line-mode fault current; The sampling sequence number is required to meet the activation criterion; The total number of sampling points for integration.

[0060] In this embodiment, the integral of the line-mode fault current is compared with two preset threshold coefficients, and the fault direction of the flexible DC power grid line is determined based on the comparison result, as follows: According to the principle analysis of the direction criterion, for a positive direction fault, such as R12, However, for faults in the opposite direction, such as R14, Taking into account noise interference, the proposed direction criterion is as follows:

[0061] ;

[0062] in, and All are threshold coefficients, and all are greater than 0. This is mainly to eliminate interference caused by noise, etc. and The value is determined based on the noise intensity and the total number of sampling points used for integration. The size is determined by.

[0063] In the direction criterion of step S2, after the start criterion is triggered, the magnitude of the fault component of the pole current is obtained by using the difference between the measured value and the steady-state value. The line-mode component and the zero-mode component are obtained by decoupling using the line-mode decoupling matrix. The positive and negative values ​​and magnitudes of the line-mode fault current integral are used to determine whether it is a positive-direction fault, a reverse-direction fault, or a disturbance.

[0064] The principle of the direction criterion in this invention is as follows: For establishing the equivalent fault circuit and analyzing fault characteristics after a DC line fault, the Peterson equivalent circuit can be used. Furthermore, for bipolar lines, there is coupling between the positive and negative poles, which can be decoupled using the symmetrical component method. The line-mode decoupling matrix is ​​then used. The fault component can be decoupled into linear mode component and zero mode component, where the decoupling matrix... for:

[0065] ;

[0066] Using Peterson's equivalent circuit and line-mode decoupling matrix You can get something like Figure 3 The equivalent circuit diagram of the fault component of the line mode after the fault is shown. Figure 3 In this context, all variables are represented in the complex frequency domain. The magnitude of the line-mode wave impedance. The magnitude of the line-mode fault voltage generated at the fault point. This represents the magnitude of the line-mode fault current at the faulty line port. This refers to the magnitude of the line-mode fault current at the non-faulty line port of the same DC bus. This represents the magnitude of the line-mode fault current flowing from the MMC to DC bus 1. This is determined by solving... Figure 3 The equivalent circuit shown can be obtained as follows:

[0067] ;

[0068] in: This is the transfer function for the fault current at the port of the faulty line. This is the transfer function for the fault current flowing from the MMC to the DC bus 1 line mode. This is the line-mode fault current transfer function at the non-faulty line port of the same DC bus. Observation reveals that... and The changes are in opposite directions, and Normally, after a line fault occurs, Therefore, the line-mode fault current measured at R12 Line-mode fault current measured at R14 By judging the magnitude of the line-mode fault current, faulty lines and non-faulty lines on the same DC bus can be distinguished. However, since the amplitude of the line-mode fault component is very small when a high-resistance fault occurs, it may be submerged in noise and other interference. This invention utilizes the integral of the line-mode fault component to improve the difference in transient characteristics between faulty lines and non-faulty lines, thereby improving the reliability of the fault direction criterion.

[0069] S3. Longitudinal Protection Criterion: After determining whether the fault direction is a forward or reverse fault, the protection device at the local signal measurement point sends the sampling point time through the communication system. The fault direction information is sent to the other end of the line; simultaneously, the protection device at the local signal measurement point receives the sampling point time from the protection device at the other end of the line. And fault direction information; when both protection devices at both ends of the line determine it as a positive fault, and the sampling point time is... and sampling point time If the judgment condition is met, the line is judged to be a faulty line; otherwise, it is a non-faulty line.

[0070] The protection device at signal measurement point R12 sends the sampling point time through the communication system. The fault direction information is sent to the other end of the line, R21. The communication system can be a private wireless network, a public wireless network, or a private fiber optic network. Each signal measurement point uses Beidou, GPS, etc. for time synchronization.

[0071] In this embodiment, when both protection devices at both ends of the line determine a positive fault, and the sampling point time is... and sampling point time If the judgment conditions are met, the line is determined to be a faulty line; otherwise, it is a non-faulty line. The judgment conditions are as follows:

[0072] ;

[0073] in, The total length of the faulty line; The estimated speed of the traveling wave propagation in the linear mode fault; For reliability coefficient, .

[0074] In the longitudinal protection criterion of step S3, after the direction criterion determines that the fault is in the positive or negative direction, the local protection device uses the communication system to send the corresponding time and fault direction information. If the fault direction information at both ends of the line is in the positive direction and the time difference is less than the time it takes for the line modulus component to pass through the entire length of the line, then the line is determined to be a faulty line.

[0075] This invention comprises three parts: a fault initiation criterion, a fault direction criterion, and a longitudinal protection criterion. It is applicable to fault protection of flexible DC grid lines and can be used as backup protection in conjunction with a single-ended ultra-fast main protection method. The fault initiation criterion utilizes the relationship between the rate of change of the pole current and a setting threshold to detect abnormal states. The fault direction criterion uses the integral of the line-mode fault current to determine forward-direction faults, reverse-direction faults, and disturbances. The longitudinal protection criterion determines whether the fault is within the line area by judging the fault direction and time difference at both ends of the line. This invention only requires measuring the current signal, facilitating deployment. It utilizes integral quantities to amplify the differences in fault characteristic quantities caused by the fault direction, resulting in clear protection boundaries, reliable identification of high-resistance faults, and strong anti-interference capabilities, meeting the requirements for practical applications.

[0076] When a high-resistance fault occurs, the DC fault current rises slowly due to the high impedance of the fault circuit, and the allowable fault protection time can be appropriately increased. Therefore, this invention uses the longitudinal protection method to identify high-resistance fault lines and cooperates with the ultra-fast main protection method to improve the reliability of flexible DC power grid line protection.

[0077] This invention proposes a fault direction criterion based on line-mode fault current integration, and then uses this criterion to propose a longitudinal protection method for flexible DC power grid faults. It aims to identify high-resistance faults in flexible DC power grid lines and can be used as backup protection in conjunction with single-ended ultra-fast main protection methods, thereby improving the reliability of flexible DC power grid line protection.

[0078] The preliminary simulation verification results of this invention are as follows: Figure 4As shown, build and in PSCAD / EMTDC Figure 2 A similar electromagnetic transient simulation model of a four-terminal mesh ±500kV flexible DC power grid, a detailed equivalent model with MMC sampling and blocking capability, a half-bridge submodule, a phase-domain frequency-dependent model for transmission lines, and a four-split conductor configuration for overhead lines with true bipolar connection were used. The simulation simulated different transitions at a distance of 60km from the R12 measurement point. For a positive ground fault, the comparison between the simulated and calculated values ​​of the fault current at the R12 position of the faulty line port is as follows: Figure 4 As shown in (a), the comparison between the simulated and calculated values ​​of the fault current at position R14 of the non-faulty line on the same DC bus is as follows: Figure 4 As shown in (b), the simulation results verify the line-mode fault current measured at R12. Line-mode fault current measured at R14 The conclusion is that the transition resistance... The difference in transient characteristics remains significant as the Ω changes from 0Ω to 400Ω. Within the integration length, the integral of the line-mode fault current obtained at R12... The integral of the line-mode fault current obtained in R14 This demonstrates that the fault direction criterion proposed in this invention is... The fault direction can be reliably identified as the resistance changes from 0Ω to 400Ω.

[0079] In flexible DC transmission networks of various voltage levels and topologies, this invention can reliably identify line faults with various transition resistances. It can be used as backup protection in conjunction with the single-ended ultra-fast main protection method, thereby improving the reliability of line protection in flexible DC power grids.

[0080] In summary, this invention proposes a longitudinal protection method for flexible DC power grid lines, aiming to overcome the difficulty of traditional single-ended ultra-fast main protection in identifying high-resistance faults in flexible DC power grids. This method utilizes the integral of the line-mode fault current to determine the fault direction; the value is positive for faults in the same direction as the DC bus and negative for faults in the opposite direction, resulting in clear protection boundaries and reliable identification of high-resistance faults. This method amplifies the difference in fault characteristic quantities caused by the fault direction by utilizing the integral quantity, possessing strong anti-interference capabilities. It can be used as a backup protection method in conjunction with single-ended ultra-fast main protection, meeting the prospects for practical applications.

Claims

1. A method of flexible DC grid line fault pilot protection, characterized in that, Comprise: Triggering criterion: Real-time measurement of the positive line current of the line where signal measurement point R12 is located in the flexible DC power grid. and negative line current According to the positive line current The rate of change of the positive line current was calculated. According to the negative line current The rate of change of the negative electrode current was calculated. The rate of change of the positive line current Rate of change of negative line current The results are compared with a preset threshold to determine whether an abnormality has occurred in the flexible DC power grid line. If an abnormality occurs, go to the next step, start the direction criterion; Direction criterion: record the sampling point time meeting the starting criterion as , and the sampling sequence number as ; The variation of the positive electrode line current and the negative electrode line current is calculated by using the steady-state current of the DC line The variation of the positive electrode line current and the negative electrode line current is calculated by using the steady-state current of the DC line The variation of the positive electrode line current and the negative electrode line current is decoupled by using the line mode decoupling matrix The variation of the positive electrode line current and the negative electrode line current is decoupled by using the line mode decoupling matrix The variation of the positive electrode line current and the negative electrode line current is decoupled by using the line mode decoupling matrix The variation of the positive electrode line current and the negative electrode line current is decoupled by using the line mode decoupling matrix According to the line mode fault current component of the fault line, calculate the line mode fault current integral; Compare the line mode fault current integral with the two preset threshold coefficients, and judge the fault direction of the flexible DC power grid line according to the comparison result; after judging the fault direction, go to the next step, start the pilot protection criterion; After judging the fault direction is positive direction fault or reverse direction fault, the protection device of local signal measurement point sends the sampling point time and fault direction information to the other end of the line through the communication system; at the same time, the protection device of local signal measurement point receives the sampling point time and fault direction information sent by the protection device of the other end of the line; when the protection devices of both ends of the line judge positive direction fault, and the sampling point time and the sampling point time meet the judgment condition, the line is judged as fault line, otherwise as non-fault line.

2. The method of flexible HVDC grid line fault pilot protection according to claim 1, characterized in that, The rate of change of the positive electrode line current The rate of change of the negative electrode line current Comparing the rate of change of the positive electrode line current with a preset threshold value, and judging whether the flexible DC power grid line is abnormal according to the comparison result. ； Wherein, is the setting threshold of the starting criterion; if the rate of change of the positive line current , the rate of change of the negative line current satisfies the above formula, it is determined that the flexible DC power grid line is abnormal, and the starting criterion direction criterion is started.

3. The method of flexible HVDC grid line fault pilot protection according to claim 1, characterized in that, The positive line current is calculated using the steady-state current of the DC line. and negative line current The change in is expressed as follows: ； ； wherein, is the amount of change in the positive line current; is the amount of change in the negative line current; is the steady state current of the DC line.

4. The method of flexible HVDC grid line fault pilot protection according to claim 1, characterized in that, The utilization line mode decoupling matrix The positive electrode line current And the negative electrode line current The change amount is decoupled to obtain the fault line mode fault current component and the zero mode fault current component, and the calculation formula is as follows: ； ； wherein, is a fault line-to-ground fault current component; is a zero-mode fault current component; is a change in positive line current; is a change in negative line current.

5. The method of flexible HVDC grid line fault pilot protection according to claim 1, characterized in that, According to the line mode fault current component of the fault line, calculate the line mode fault current integral, and the calculation formula is as follows: ； wherein is the line mode fault current integral; is the sample number that meets the start criterion; is the total number of sample points for the integral.

6. The method of flexible HVDC grid line fault pilot protection according to claim 1, characterized in that, According to the line mode fault current component of the fault line, calculate the line mode fault current integral, and the calculation formula is as follows: ； wherein and are threshold coefficients, and are both greater than 0.

7. The method of flexible HVDC grid line fault pilot protection according to claim 1, characterized in that: The threshold coefficient and The value is determined based on the noise intensity and the total number of sampling points used for integration. Decide.

8. The method of flexible HVDC grid line fault pilot protection according to claim 1, characterized in that: When both the line-end protection devices judge the fault as a positive direction fault, and the sampling point time and the sampling point time satisfies the judgment condition, the line is judged as a fault line, otherwise, it is a non-fault line, wherein the judgment condition is as follows: ； wherein, is the full length of the faulty line; is the estimated speed of the line mode fault traveling wave; is the reliability factor, .

9. The method of flexible HVDC grid line fault pilot protection according to claim 1, characterized in that: The protection method is suitable for single-pole grounding fault and double-pole short circuit fault.

10. The method of flexible HVDC grid line fault pilot protection according to claim 1, characterized in that: The communication system is a wireless private network, a wireless public network or an optical fiber private network.

Images