A multi-terminal flexible direct current power grid transmission line borderless protection method and system

Abstract

A kind of multi-terminal flexible DC power grid transmission line no-boundary protection method and system, the characteristic frequency range of reverse voltage traveling wave fault component of fault line mode in no-boundary transmission line area is acquired in advance;Positive voltage and negative voltage at the protection installation of multi-terminal flexible DC transmission line are collected, and protection starting detection is realized using the size of positive and negative sudden change, and cumulative zero-mode voltage is used to realize fault pole selection;The absolute value of the ratio of line mode forward voltage traveling wave fault component and line mode reverse voltage traveling wave fault component is compared with the direction threshold to determine the fault direction;Using line mode voltage fault component, line mode current fault component and wavelet packet transform, phased fault area detection is realized.The present application can make I section cut off serious fault in DC line area within 1x10 ‑3 Second, II section can cut off fault at any position in DC line area within 10x10 ‑3 Second.

Description

Technical Field

[0001] This invention relates to a boundaryless protection method for multi-terminal flexible DC power grid transmission lines based on high-frequency components of reverse traveling waves, belonging to the field of transmission line protection in relay protection. Background Technology

[0002] For nearly a century, global warming, sea-level rise, and other environmental problems caused by excessive emissions of greenhouse gases such as carbon dioxide have become increasingly prominent. Promoting a clean and low-carbon transition in energy consumption has become a key path to achieving the goals of "carbon peaking" and "carbon neutrality." In the existing energy structure, the power system occupies a central position; therefore, developing a new power system based primarily on new energy sources has become an important direction for the transformation of the power industry. To enhance the power system's capacity to accommodate new energy sources, flexible DC grid technology has emerged.

[0003] Currently, flexible DC grid technology is still under development and faces several key challenges. Compared to AC grids, flexible DC grids exhibit low inertia, with current rising rapidly and reaching high peak values ​​during faults. However, the power electronic devices in converter stations have limited current-carrying capacity, thus imposing millisecond-level stringent requirements on the response speed of the protection system. Currently, most multi-terminal flexible DC projects suppress short-circuit current levels and establish boundary conditions between adjacent lines by configuring current-limiting reactors at both ends of the transmission lines, providing a decoupling path for protection selectivity. Scholars have proposed various protection strategies based on this structure. However, with the expansion of the grid coverage, deploying current-limiting reactors line-by-line would significantly increase construction costs, hindering economic optimization. Therefore, concentrating current-limiting reactors at the converter station outlet better balances current-limiting performance and economic feasibility. However, this arrangement eliminates boundary elements on each line, introducing new technical challenges to protection selectivity—the need to identify fault sections in the absence of boundary assistance.

[0004] Currently, protection methods can be broadly categorized into two types based on whether they rely on peer information: two-terminal and single-terminal. In two-terminal protection, some studies have proposed boundaryless protection strategies based on the traveling wave differential principle; however, this scheme requires synchronous transmission of voltage and current data at each sampling moment, placing extremely high demands on the communication system. Another method utilizes the time difference between the arrival of the forward and reverse traveling waves at the protection unit to identify the fault location, but its effectiveness is significantly affected by the length of adjacent lines. In single-terminal protection, one scheme extracts the exponential features from the fault voltage propagation term using a fitting algorithm to construct a single-terminal protection criterion; however, the Levenberg-Marquardt algorithm used is extremely sensitive to initial values, leading to insufficient stability. Other research attempts to fit parameters reflecting fault characteristics from the zero-mode current to obtain the transition resistance estimation range and construct an adaptive traveling wave protection mechanism; however, due to its complex algorithm and reliance on high sampling rates, its practical application in engineering is challenging.

[0005] Therefore, this invention proposes a method for the protection of boundaryless transmission lines in multi-terminal flexible DC power grids based on the high-frequency component of reverse traveling waves, aiming to more effectively simplify the protection method for boundaryless transmission lines in multi-terminal flexible DC power grids. Summary of the Invention

[0006] To address the technical challenge of efficiently and rapidly identifying fault sections in multi-terminal flexible DC power grids without boundary assistance, this invention discloses a boundaryless protection method and system for transmission lines in multi-terminal flexible DC power grids.

[0007] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows: The first aspect of this invention discloses a boundaryless protection method for multi-terminal flexible DC power grid transmission lines, comprising: S1: Analyze the amplitude-frequency difference of faults inside and outside the boundaryless transmission line area of ​​the multi-terminal flexible DC grid, and obtain the characteristic frequency range of the line-mode reverse voltage traveling wave fault component inside the area; S2: Real-time acquisition of positive voltage, negative voltage, positive current, and negative current at the installation location of the multi-terminal flexible DC power grid boundaryless transmission line protection for single-ended quantity protection; S3: Calculate the voltage change based on the positive and negative voltages, use the voltage change to detect the protection start-up, and record the disturbance time value after the protection starts. S4: Set the disturbance time interval based on the disturbance moment, calculate the accumulated zero-mode voltage for fault detection; S5: Based on the real-time collected positive voltage, negative voltage, positive current, and negative current, calculate the line-mode forward voltage traveling wave fault component and the line-mode reverse voltage traveling wave fault component within the disturbance time interval. Determine the fault direction by comparing the absolute value of the ratio of the line-mode forward voltage traveling wave fault component to the line-mode reverse voltage traveling wave fault component with a direction threshold, and obtain the fault direction index. D r ; S6: Based on the pre-acquired characteristic frequency range of the line-mode reverse voltage traveling wave fault component within the region, perform wavelet packet transform on the line-mode reverse voltage traveling wave fault component to obtain the maximum value of the directional high-frequency voltage. DF ,like DF If the voltage exceeds the first high-frequency voltage threshold, it is determined to be an intra-zone fault; if DF If the voltage is less than or equal to the second high-frequency voltage threshold, it is judged as an external fault; otherwise, the line-mode voltage change is obtained after a delay and the normalized line-mode voltage value is obtained. When the cross-correlation coefficient between the normalized line-mode voltage value and the set positive polarity step signal is less than 0, it is judged as an internal fault; otherwise, it is an external fault.

[0008] More preferably, In S1, the characteristic frequency range of the reverse voltage traveling wave fault component in the fault line mode within the region is obtained in advance using the reverse traveling wave analysis method. For any location not at the beginning or end of the line, faults at the end of the line, faults at the beginning of the line, and any location not at the beginning or end of an adjacent line, an expression for the reverse voltage traveling wave fault component is established, and the line-mode reverse voltage traveling wave fault component is solved. The amplitude-frequency curves of the line-mode reverse voltage traveling wave fault component under different fault locations are plotted to obtain the characteristic frequency range of the line-mode reverse voltage traveling wave fault component within the transmission line area.

[0009] More preferably, In S3, based on the collected positive voltage... U p ( t ), negative voltage U n ( t Average rated voltage of the positive pole of a multi-terminal flexible DC power grid U rp Average rated voltage of the negative pole of a multi-terminal flexible DC power grid U rn Calculate the corresponding positive fault sudden voltage respectively. U pm ( t ) and negative electrode fault sudden voltage U nm ( t ):

[0010]

[0011] In the formula, t Time is measured in seconds, and its symbol is s. When detected U pm ( t )or U nm ( t (Greater than the rated voltage of the positive terminal of a multi-terminal flexible DC power grid) U ra of sx If the value is multiple times the threshold, it is determined that a disturbance has occurred in the multi-terminal flexible DC power grid, the protection system is activated, and the value at that moment is recorded. t fs As the time value of the disturbance.

[0012] More preferably, Determined based on the principle that even the largest possible single-pole grounding fault transition resistance in a multi-terminal flexible DC power grid can ensure normal startup. sx The value of .

[0013] More preferably, In S4, the accumulated zero-mode voltage is calculated according to the following formula:

[0014] when When a positive ground fault is detected, it is determined that a ground fault has occurred; when If the fault occurs, it is determined that a negative grounding fault has occurred; otherwise, it is determined that a double grounding fault has occurred. in, For the time of disturbance, This refers to the disturbance time interval set based on the disturbance time. , These are the pre-extension time and the post-extension time, respectively. The zero-mode voltage is calculated over the disturbance time interval. This is the threshold for zero-mode voltage accumulation.

[0015] More preferably, The zero-mode voltage accumulation threshold U jx The tuning calculation is performed according to the following formula.

[0016] In the formula, k jx For fault selection, the reliability coefficient is used. f s The signal sampling frequency, T w For signal time window, U ra This is the rated voltage of the positive terminal of the power grid.

[0017] More preferably, Select Second; Second; the symbol for a second is s; in f s When the frequency is 100kHz, select k jx It is 0.01.

[0018] More preferably, In S5, the fault direction is determined based on the following criteria to obtain the fault direction index. D r : Forward fault: ; Reverse fault: ; in, , These represent the forward voltage traveling wave fault component and the reverse voltage traveling wave fault component of the line mode within the disturbance time interval, respectively. The set direction threshold.

[0019] More preferably, Linear mode forward voltage traveling wave fault component and line mode reverse voltage traveling wave fault component Calculate using the following formula:

[0020]

[0021] In the formula, and These represent the fault components of line-mode voltage and line-mode current during the disturbance time interval, respectively. This represents the mode impedance of the transmission line.

[0022] More preferably, In S6, the fault component Δ of the line-mode reverse voltage traveling wave is... u B Wavelet packet decomposition is performed. Based on the characteristic frequency range of the reverse voltage traveling wave fault component of the fault line mode within the transmission line area, the wavelet packet is selected according to the frequency range from small to large. cs Layer wavelet packet reconstruction coefficients are used to obtain the high-frequency traveling wave fault component Δ. u HB Then, the maximum value of the reverse high-frequency voltage is calculated according to the following formula. DF :

[0023] Where, Δ u HDB For directional high-frequency voltage, DF The maximum value of the high-frequency voltage in the direction after the disturbance moment. cs The selection is made by combining the sampling frequency of the traveling wave protection signal and the characteristic frequency range of the reverse voltage traveling wave fault component of the fault line mode in the transmission line area obtained in advance.

[0024] More preferably, When the fault is a single-pole fault: The first high-frequency voltage threshold should be greater than that of a near-field metallic unipolar fault outside the fault zone. DF 1.1 times the value; The range of the second high-frequency voltage threshold is selected based on the operating condition of the DC circuit breaker's energy-consuming branch when it cannot start under a single-pole fault. When the fault is a bipolar fault: The first high-frequency voltage threshold value should be greater than that of a near-field metallic bipolar fault outside the fault zone. DF1.1 times the value; The range of the second high-frequency voltage threshold is selected based on the operating condition of the DC circuit breaker's energy-consuming branch when it cannot start under bipolar fault.

[0025] More preferably, In S6, when the directional high-frequency voltage reaches its maximum value... DF When the voltage is greater than the second high-frequency voltage threshold but less than the first high-frequency voltage threshold. Obtain the line-mode voltage change Δ at the protection installation location. U 1. Obtain the normalized line-mode voltage Δ GU 1. Find Δ GU 1 and positive polarity step signal J ref ( t Cross-correlation coefficients between ) CF ,like CF If the value is less than 0, it is determined to be a fault within the area; if CF If the value is greater than 0, it is determined to be an external fault.

[0026] A second aspect of this invention discloses a boundaryless protection system for multi-terminal flexible DC power grid transmission lines utilizing the aforementioned boundaryless protection method, the system comprising: The reverse voltage traveling wave fault component analysis module analyzes the amplitude-frequency difference of faults inside and outside the boundaryless transmission line of the multi-terminal flexible DC grid based on the system topology and transmission line parameters, and obtains the characteristic frequency range of the reverse voltage traveling wave fault component of the line mode inside the boundary. The single-ended quantity information acquisition module is used to acquire the positive voltage, negative voltage, positive current, and negative current at the installation location of the multi-terminal flexible DC power grid boundaryless transmission line protection for single-ended quantity protection. The protection start detection module calculates the voltage change based on the positive and negative voltages, uses the voltage change to detect the protection start, and records the disturbance time value after the protection starts. The fault polarity detection module calculates and accumulates the zero-mode voltage to detect the fault polarity based on the disturbance time interval set at the disturbance time. The fault direction determination module determines the fault direction by comparing the absolute value of the ratio of the forward voltage traveling wave to the reverse voltage traveling wave of the line mode with a direction threshold, and obtains the fault direction index to determine the fault direction. The fault area detection module, based on the pre-acquired characteristic frequency range of the line-mode reverse voltage traveling wave fault component within the area, performs wavelet packet transform on the line-mode reverse voltage traveling wave to obtain the maximum value of the reverse high-frequency voltage. DF ,like DF If the voltage exceeds the first high-frequency voltage threshold, it is determined to be an intra-zone fault; if DFIf the voltage is less than or equal to the second high-frequency voltage threshold, it is judged as an external fault; otherwise, the line-mode voltage change is obtained after a delay and the normalized line-mode voltage value is obtained. When the cross-correlation coefficient between the normalized line-mode voltage value and the set positive polarity step signal is less than 0, it is judged as an internal fault; otherwise, it is an external fault.

[0027] The third aspect of this invention discloses a computer device, including a memory, a processor, and a computer program stored in the memory; characterized in that: the processor executes the computer program to implement the steps of the multi-terminal flexible DC power grid transmission line boundaryless protection method.

[0028] The fourth aspect of the present invention discloses a computer-readable storage medium having a computer program stored thereon, characterized in that the computer program, when executed by a processor, implements the steps of the method for boundaryless protection of multi-terminal flexible DC power grid transmission lines.

[0029] Compared with the prior art, the present invention has the following beneficial technical effects: 1. This invention obtains the characteristic frequency range of the fault component of the reverse traveling wave of the fault line mode voltage in the boundaryless transmission line area of ​​a multi-terminal flexible DC power grid with different fault locations through reverse traveling wave analysis theory. 2. A directional high-frequency voltage maximum value quantization index was constructed using wavelet packet transform and fault direction to achieve quantitative identification of faults inside and outside the fault zone. This realized a single-ended two-stage protection method independent of boundary characteristics. The fault zone detection criterion for stage I can be implemented within 1×10-1. -3 Severe faults within the DC line area can be cleared within seconds; the fault area detection criterion for Section II can be 7×10 -3 Faults at any location within the DC line area can be cleared within seconds. Attached Figure Description

[0030] Figure 1 This is a flowchart of a method for boundaryless protection of multi-terminal flexible DC power grid transmission lines according to an embodiment of the present invention; Figure 2(a) is a schematic diagram of the system topology of the embodiment; Figure 2(b) is a schematic diagram of the fault points in the embodiment; Figure 3 This is the equivalent circuit diagram for a fault at any location other than the beginning or end of the line. Figure 4 This is the equivalent circuit diagram of a fault occurring at the end of this line. Figure 5 This is the equivalent circuit diagram of a fault occurring at the beginning of this line. Figure 6 This is the equivalent circuit diagram for a fault that occurs at any point other than the beginning or end of an adjacent line. Figure 7 High-frequency component diagrams of the reverse traveling wave under different fault distances and transition resistances; Figure 8 This is a typical hybrid DC circuit breaker topology diagram; Figure 9 A schematic diagram of the line-mode traveling wave of the operating voltage of a hybrid DC circuit breaker; Figure 10 This is a diagram of a boundaryless topology for a multi-terminal flexible DC power grid. Figure 11 High-frequency component diagrams of the reverse traveling wave at different fault distances; Figure 12 Diagram showing the ability of section I to withstand transition resistance; Figure 13 Diagrams showing the electrical quantities involved in protection 7 under different operating conditions; Figure 14 Diagrams showing the electrical quantities involved in protection 5, protection 6, and protection 7 under different operating conditions; Figure 15 It is a positive polarity step signal. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of this invention. The embodiments described in this application are merely some embodiments of this invention, and not all embodiments. Based on the spirit of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of this invention.

[0032] like Figure 1 As shown, a boundaryless protection method for multi-terminal flexible DC power grid transmission lines based on high-frequency components of reverse traveling waves includes the following steps: A boundaryless protection method for multi-terminal flexible DC power grid transmission lines based on high-frequency components of reverse traveling waves includes the following steps: Step 1: Based on the system topology and transmission line parameters, use the reverse traveling wave analysis method to analyze the amplitude-frequency difference of faults inside and outside the boundaryless transmission line of the multi-terminal flexible DC grid, and obtain the characteristic frequency range of the reverse voltage traveling wave fault component of the line mode inside the area; Step 1.1: Obtain the reverse voltage traveling wave fault component of the line mode when the fault occurs at any position other than the beginning or end of the line. expression:

[0033] In the formula, This refers to the reverse voltage traveling wave fault component of the line mode when the fault occurs at any location other than the beginning or end of the line. Z 1c The mode impedance of the transmission line. Z 0cFor the zero-mode impedance of the transmission line, U dc+ The system's rated voltage. For attenuation components, R g For transition resistance, γ A1 This is the propagation coefficient of the line mode. x Distance to the fault; Step 1.2: Obtain the expression for the reverse voltage traveling wave fault component when the fault occurs at the end of this line:

[0034]

[0035] In the formula, Z represents the line-mode reverse voltage traveling wave fault component when the fault occurs at the end of this line. 1Σ Z is the equivalent line-mode impedance of the system seen from the fault point. 0Σ The equivalent zero-mode impedance of the system as seen from the fault point. Z 0s For the zero-mode impedance of the converter station, Z 1s The converter station line-mode impedance; Step 1.3: Obtain the expression for the reverse voltage traveling wave fault component when the fault occurs at the beginning of this line:

[0036]

[0037] In the formula, This is the line-mode reverse voltage traveling wave fault component when the fault occurs at the beginning of this line. Step 1.4: Obtain the expression for the reverse voltage traveling wave fault component when the fault occurs at a point other than the beginning or end of an adjacent line:

[0038] In the formula, γ B1 The propagation coefficient of the adjacent line mode. ρ B The refractive index is 1. To protect P B The equivalent impedance at that point.

[0039] Step 1.5: Using typical parameters of the transmission line, plot the amplitude-frequency diagram of the reverse voltage traveling wave fault component at different fault locations, and determine the characteristic frequency range of the reverse voltage traveling wave fault component of the fault line mode in the area based on the amplitude-frequency diagram.

[0040] Step 2: Real-time acquisition of positive voltage, negative voltage, positive current, and negative current at the installation location of the multi-terminal flexible DC power grid boundaryless transmission line protection with single-ended quantity protection; Step 3: Calculate the voltage change based on the positive and negative voltages, use the voltage change to detect the protection start-up, and record the disturbance time value after the protection starts. Based on the collected positive voltage U p ( t ), negative voltage U n ( t (), Rated positive voltage of multi-terminal flexible DC power grid U rp The system's negative terminal rated voltage U rn Calculate the corresponding positive fault sudden voltage for each of them. U pm ( t ) and negative electrode fault sudden voltage U nm ( t ), U pm ( t )and U nm ( t The calculation method is as follows:

[0041]

[0042] In the formula, t Time is measured in seconds, and its symbol is s. f s Sampling frequency, t d For the current moment; Based on the above formula, the protection start detection criterion can be constructed as follows:

[0043]

[0044] In the formula, U p and U n These are the positive and negative voltages. U rp and U rn The system's rated voltage can be used. U set For the system rated voltage sx times; If the protection activation detection criteria are met, the protection is determined to be activated; to improve the speed and sensitivity of the activation detection criteria of the proposed protection method, this invention patent sets... sx The value is 0.03. This setting principle is based on the assumption that the maximum possible single-pole grounding fault transition resistance in a multi-terminal flexible DC grid can also be activated. For example, taking a 500kV flexible DC grid as an example, the maximum single-pole grounding fault transition resistance found in existing research is selected as 1500Ω.

[0045] Step 4: Set the disturbance time interval based on the disturbance time, and calculate the accumulated zero-mode voltage for fault detection; Step 4.1 Utilize U pf ( t )and U nf ( t To calculate the zero-mode voltage U Z ( t and accumulated zero-mode voltage U ZO ,in, U pf ( t )and U nf ( t The time interval is [ t fs -0.1×10 -3 Second, t fs +0.3×10 -3 Second], U Z ( t )and U ZO The calculation method is as follows:

[0046]

[0047] Step 4.2: If U ZO Less than the voltage accumulation threshold - U jx If so, it is determined to be a positive ground fault; if U ZO Greater than the voltage accumulation threshold U jx If the fault is between - U jx and U jx If the distance is between, then a bipolar fault is determined. Ujx The tuning calculation can be performed according to the following formula, where k jx For fault selection, the reliability coefficient is used. f s The signal sampling frequency, T w For signal time window, U ra This is the rated voltage of the positive terminal of the power grid.

[0048]

[0049] Select Second; seconds; in f s When the frequency is 100kHz, select k jx It is 0.01; calculated as follows U jx The value is 200kV.

[0050] Step 5: Based on the real-time collected positive voltage, negative voltage, positive current, and negative current, calculate the forward and reverse voltage traveling wave fault components of the line mode within the disturbance time interval. Determine the fault direction by comparing the absolute value of the ratio of the forward to reverse voltage traveling wave fault components with the direction threshold, and obtain the fault direction index. D r ; Step 5.1: Obtain the line-mode voltage fault component Δ U 1 and line-mode current fault component Δ I 1, Δ U 1 and Δ I The expression for 1 is:

[0051]

[0052] In the formula, U p and U n The voltage between the positive and negative terminals. U 1 and I 1 represents the line-mode voltage and line-mode current. I p and I n These are the positive and negative currents. U mm for U 1 in t fs Previously 0.1×10 -3Average value per second I mm for I 1 in t fs Previously 0.1×10 -3 The average value over seconds; additionally, the line-mode voltage fault component Δ U The time window of 1 is [ t fs -0.1×10 -3 Second, t fs +0.3×10 -3 [seconds], line-mode current fault component Δ I The time window of 1 is also [ t fs -0.1×10 -3 Second, t fs +0.3×10 -3 Second]; Step 5.2: Obtain the fault component Δ of the traveling wave of the line mode forward voltage. u F and the reverse voltage traveling wave fault component Δ of the line mode u B Their expressions are as follows:

[0053]

[0054] In the formula, Δ U 1 and Δ I 1 represents the fault components of line-mode voltage and line-mode current. Z 1 represents the mode impedance of the transmission line; Step 5.3: Obtain fault direction indicators D r The expression is: Forward fault: ; Reverse fault: ; Existing theoretical research indicates that when a forward fault occurs, Δ u F The amplitude is much smaller than Δ u B Its absolute value of amplitude will be less than 1. To ensure a certain discrimination margin, select K set The value is 2.

[0055] Step 6: Based on the pre-acquired characteristic frequency range of the line-mode reverse voltage traveling wave fault component within the region, perform wavelet packet transform on the line-mode reverse voltage traveling wave fault component to obtain the maximum value of the directional high-frequency voltage. DF ,like DF If the voltage exceeds the first high-frequency voltage threshold, it is determined to be an intra-zone fault; if DF If the voltage is less than or equal to the second high-frequency voltage threshold, it is judged as an external fault; otherwise, the line-mode voltage change is obtained after a delay and the normalized line-mode voltage value is obtained. When the cross-correlation coefficient between the normalized line-mode voltage value and the set positive polarity step signal is less than 0, it is judged as an internal fault; otherwise, it is an external fault. First, the maximum value of the high-frequency voltage in the direction is obtained to determine the fault area detection criteria for segment I. Step 6.1: For the reverse traveling wave Δ u B Wavelet packet decomposition is performed, with db3 as the wavelet basis, and the number of decomposition levels is [not specified]. nn You can get 2 nn Based on the characteristic frequency range of the reverse voltage traveling wave fault component of the fault line mode within the transmission line area, the first time-frequency component is selected according to the frequency band range from small to large. cs Layer wavelet packet reconstruction coefficients are high-frequency reverse traveling wave Δ u HB ; cs The selection is based on the pre-obtained characteristic frequency range and the 100kHz sampling frequency commonly used in traveling wave protection. In this invention, the number of decomposition layers is selected as 3. cs The value of 8 was selected, and the wavelet basis was chosen according to the wavelet basis commonly used in existing DC transmission line protection methods. Step 6.2: Obtain the directional high-frequency voltage Δ u HDB And set its maximum value after startup to be DF ;Δ u HDB and DF The calculation method is as follows:

[0056] Where, Δ u HDB For directional high-frequency voltage, DF The maximum value of the high-frequency voltage in the direction after the disturbance moment; Step 6.3: Determine DF Is it greater than the first high-frequency voltage threshold? HB 1ty If so, the fault is determined to be a fault within the protection zone; if DF Less than or equal to the second high-frequency voltage threshold HB 2ty If so, the fault is determined to be an external fault within the protection zone. Conversely, if DF Greater than the second high-frequency voltage threshold HB 2ty But less than or equal to the first high-frequency voltage threshold HB 1ty If so, proceed to step 6.4 to determine the fault area detection criteria for stage II; Step 6.4: Analyze the action time according to the fault area detection criteria for segment I. t p1 Operating time of hybrid high voltage DC circuit breakers t b1 To obtain the line-mode voltage change Δ at the protection point U The starting time of 1 is determined, and its time window is set to 0.5 × 10⁻⁶. -3 Second; Step 6.5: For Δ U 1. Perform a normalization operation, with the maximum and minimum values ​​of the normalization range being 1 and -1, respectively, to obtain the normalized line-mode voltage Δ. GU 1; Step 6.6: Based on the fact that the fault line mode voltage within the zone exhibits negative polarity and the fault line mode voltage outside the zone exhibits positive polarity, a positive polarity step signal can be set. J ref ( t The amplitude range is 0 to 1. To avoid boundary effects, the step point can be selected based on 1 / 5 of the data length of the line-mode voltage change. Step 6.7: Calculate Δ GU 1 and J ref ( t Cross-correlation coefficients between ) CF ,like CF If the value is less than 0, it is determined to be a fault within the area; if CF If the value is greater than 0, it is determined to be an external fault.

[0057] To analyze the single-ended quantity protection method of the present invention that does not depend on boundary characteristics, the topology diagram shown in Figure 2(a) and the fault point diagram shown in Figure 2(b) are used.

[0058] In Figure 2(a), it is assumed that the line wave impedance is the same and the length is 500km. P A P B These are the installation locations for protection on this line and adjacent lines, respectively. See Figure 2(b). f 1 indicates a fault at any point other than the beginning or end of the line. f 2 indicates a fault at the beginning of the adjacent line. f 3 indicates a fault at any point other than the beginning or end of an adjacent line. f 4 indicates a fault at the beginning of the line. See also... Figure 3 , Z c For line wave impedance, Z s This includes the converter's equivalent impedance and current-limiting reactance impedance. Uf For metallic faults, the equivalent traveling wave source is... R g This is the transition resistance between the fault point and ground. U ' f The fault point is the equivalent traveling wave source. Parameters such as line unit impedance, admittance, converter capacitance, number of submodules, bridge arm reactance, and current-limiting reactance can be adopted from the Zhangbei Flexible DC System parameters.

[0059] Because the traveling wave method typically uses fault components to construct protection criteria, it calculates zero-mode fault components and line-mode fault components. Based on traveling wave analysis theory, the zero-mode reverse voltage traveling wave fault component can be obtained. and line mode reverse voltage traveling wave fault component as follows:

[0060] In the formula, The equivalent line-mode impedance of the system as seen from the fault point. The equivalent zero-mode impedance of the system as seen from the fault point. u fa0 It is a zero-mode reverse voltage traveling wave. u fa1 For line-mode reverse voltage traveling wave, u a1 The reverse voltage traveling wave of the line mode before the fault. U dc+ The rated voltage of the positive terminal of the power grid. R g For the transition resistance, .

[0061] When an incident occurs at any location other than the beginning or end of this line... f 1. When a fault occurs, Figure 3 The dashed box shows the equivalent circuit diagram as seen from the fault point, specifically the part inside the dashed box.

[0062] based on Figure 3 It can provide the system's equivalent line-mode impedance that allows us to see the fault point. and zero-mode impedance The voltage reverse traveling wave line mode fault component can be obtained as shown in the following formula.

[0063]

[0064]

[0065] In the formula, γ A1 This is the propagation coefficient of the local line mode. x The fault location distance protection P on this line A The distance.

[0066] When the fault occurs at the end of this line, the equivalent circuit diagram is shown below. Figure 4 Dashed box.

[0067] based on Figure 4 It can provide the system's equivalent line-mode impedance that allows us to see the fault point. and zero-mode impedance As shown in the following formula.

[0068]

[0069] Based on the above equation, the voltage reverse traveling wave line mode fault component is further obtained as shown in the following equation.

[0070] In the formula, γ A1 This is the propagation coefficient of the local line mode. x Take the total length of this route l , Z 0s For the zero-mode impedance of the converter station, Z 1s The converter station line-mode impedance; In addition, since it is a borderless network, the fault at the beginning of the adjacent line is the same as the fault at the end of this line.

[0071] When the fault occurs at the beginning of this line, the equivalent circuit diagram is shown below. Figure 5 Dashed box. Based on Figure 5 It can provide the system's equivalent line-mode impedance that allows us to see the fault point. and zero-mode impedance See the following formula:

[0072] Further, the voltage reverse traveling wave line mode fault component is obtained as shown in the following formula:

[0073] When the fault occurs at a point other than the beginning or end of an adjacent line, the equivalent circuit diagram is shown below. Figure 6 Dashed box. Based on Figure 6 It can provide the system's equivalent line-mode impedance at the point of failure. and zero-mode impedance Furthermore, considering the reflection and refraction behavior that occurs when a traveling wave reaches a discontinuity in wave impedance, ρ B Let be the refractive index. The following formula can be obtained: P A The voltage at the location is a reverse traveling wave line mode fault component.

[0074]

[0075]

[0076] In the formula, This is the propagation coefficient of the local line mode. The propagation coefficient of the adjacent line mode. Z sPB To protect P B The equivalent impedance at the point; Based on the above analysis, using typical parameters of a multi-terminal flexible DC power grid project, and selecting the frequency of the characteristic signal as 45.2kHz, we can further draw... Figure 7 The diagram shows the high-frequency components of the reverse voltage traveling wave under different fault distances and transition resistances. Figure 7 It is known that as the transition resistance increases, the amplitude of the high-frequency component of the reverse traveling wave within the protection zone under a large transition resistance is smaller than that outside the protection zone under a small transition resistance. For example, under a 300Ω transition resistance, using the amplitude of the high-frequency component of the near-region metallic reverse traveling wave outside the protection zone as a benchmark, and after taking a certain reliability coefficient, a portion of the line length can be protected. This indicates that relying solely on the amplitude of the high-frequency component of the reverse traveling wave cannot achieve effective protection for the entire length of the line.

[0077] Therefore, to protect the entire line length, the amplitude of the high-frequency component of the reverse voltage traveling wave is first used to construct the fault zone detection criterion for Section I. The selection principle for the setting of Section I is to multiply the amplitude of the high-frequency component of the reverse voltage during near-zone metallic faults outside the protection zone by a certain reliability coefficient. That is, Section I sacrifices a certain line protection length to ensure fast operation. Theoretically, the operating time of Section I will consist of the criterion data calculation time window and a certain delay.

[0078] Therefore, in order to protect the entire line length, the setting of the protection method for the amplitude of the high-frequency component of the reverse traveling wave is extended to the downstream line, and with the corresponding delay and auxiliary criteria, a second-stage fault area detection criterion based on the high-frequency component of the reverse voltage traveling wave is formed.

[0079] According to existing research, in multi-terminal flexible DC grid ring network structures, installing high-voltage DC circuit breakers (HVDC circuit breakers) is an effective solution to ensure selective fault clearing. In this invention, regardless of the type of HVDC circuit breaker, a metal oxide varistor (MOV) is used to construct an energy-dissipating branch for fault energy dissipation. A typical hybrid high-voltage DC circuit breaker (DCCB) topology is shown below. Figure 8 As shown.

[0080] This section further explains the working principle of a hybrid high-voltage DC circuit breaker. Under normal conditions, the main branch carries the system current with minimal conduction losses. Upon receiving a trip command, the main branch auxiliary transfer module disconnects, and the current flows into the transfer branch. After the main branch's fast mechanical switch restores its insulation capability, the DCCB disconnects the transfer branch's power electronic module, and the current flows into the energy-consuming branch. The energy-consuming branch's MOV (Mechanical Operating Voltage) generates a transient breaking voltage to suppress fault current. This transient breaking voltage also propagates along the transmission line as a voltage traveling wave. This is the fundamental reason for the generation of the operating voltage traveling wave in a DC circuit breaker. The propagation process of the linear modulus component of the operating voltage traveling wave is analyzed below.

[0081] Taking the fault in the R3 protection zone and the operation of its hybrid DC circuit breaker as an example, Figure 9 A schematic diagram of the line-mode traveling wave of the operating voltage of the hybrid DC circuit breaker within the R3 zone is given. Figure 9 In this context, the direction from the busbar to the line is considered positive, and the opposite direction is considered negative. In the event of a fault... f After step 2, a negative polarity traveling wave fault component will be generated at the fault point. This negative polarity traveling wave fault component will propagate along the transmission line. At this time, the traveling wave fault components measured by R1, R2, R3, and R4 will also show negative polarity, that is, the line-mode voltage will show a decreasing trend. Next, when the fault within the zone is detected at R3, the hybrid DC circuit breaker at R3 will operate. When its energy-consuming branch reaches the starting voltage, a positive polarity operating voltage traveling wave superposition component will be generated at the hybrid DC circuit breaker. Regarding this operating voltage traveling wave superposition component, it can be discussed by region. Taking R4 with the fault within the zone as an example, since the fault point still exists, R4 will still experience the negative polarity traveling wave fault component. Taking R1 and R2 with the fault outside the zone as an example, since the hybrid DC circuit breaker has completed the fault current interruption, R1 and R2 are no longer affected by the fault point. The traveling waves measured at R1 and R2 are positive polarity operating voltage traveling wave superposition components, and their voltage will show an increasing trend. Therefore, fault area detection can be achieved based on the difference in voltage trends inside and outside the fault zone.

[0082] This invention also discloses a boundaryless protection system for multi-terminal flexible DC power grid transmission lines, the system comprising: The reverse voltage traveling wave fault component analysis module analyzes the amplitude-frequency difference of faults inside and outside the boundaryless transmission line of the multi-terminal flexible DC grid based on the system topology and transmission line parameters, and obtains the characteristic frequency range of the reverse voltage traveling wave fault component of the line mode inside the boundary. The single-ended quantity information acquisition module is used to acquire the positive voltage, negative voltage, positive current, and negative current at the installation location of the multi-terminal flexible DC power grid boundaryless transmission line protection for single-ended quantity protection. The protection start detection module calculates the voltage change based on the positive and negative voltages, uses the voltage change to detect the protection start, and records the disturbance time value after the protection starts. The fault polarity detection module calculates and accumulates the zero-mode voltage to detect the fault polarity based on the disturbance time interval set at the disturbance time. The fault direction determination module determines the fault direction by comparing the absolute value of the ratio of the forward voltage traveling wave to the reverse voltage traveling wave of the line mode with a direction threshold, and obtains the fault direction index to determine the fault direction. The fault area detection module, based on the pre-acquired characteristic frequency range of the line-mode reverse voltage traveling wave fault component within the area, performs wavelet packet transform on the line-mode reverse voltage traveling wave to obtain the maximum value of the reverse high-frequency voltage. DF If the voltage is greater than the first high-frequency voltage threshold, it is judged as an in-zone fault; if it is less than or equal to the second high-frequency voltage threshold, it is judged as an out-of-zone fault; otherwise, the line-mode voltage change is obtained after a delay and the normalized line-mode voltage value is obtained. When the cross-correlation coefficient between the normalized line-mode voltage value and the set positive polarity step signal is less than 0, it is judged as an in-zone fault; otherwise, it is an out-of-zone fault.

[0083] Example: In this invention, a borderless four-terminal ring flexible DC power grid with a rated voltage of ±500kV and a length of 500km for each transmission line is used as an example, taking the case of a near-zone metallic fault outside the zone as an example. DF 1.1 times HB 1ty The setting is based on the condition that the power-consuming branch of the DC circuit breaker cannot start. HB 2ty The setting, and further, when the fault is an extremely unipolar fault, HB 1ty The value can be taken as 1.37kV. HB 2ty The value can be taken as 0.65kV; when the fault is a bipolar fault, the same selection method applies. HB 1ty 4kV can be taken as the value. HB 2ty The voltage can be set to 2.3kV; simultaneously, the time window for the I-segment signal is 0.1 × 10⁻⁶ kV before the fault. -3 Seconds and 0.3 × 10 after the fault -3 Seconds, plus the inherent delay of segment I, 0.5 × 10. -3 The typical operating time of hybrid high-voltage DC circuit breakers is 3 × 10 seconds. -3 Within seconds; therefore, we can let Δ t dd 3.8×10 -3 Second.

[0084] Select Figure 10The multi-terminal flexible DC grid topology shown has the following lines. l 1. l 2. l 3 and l The lengths of the four stations are 500km, 500km, 500km and 500km respectively, and their specific parameters are shown in Table 1.

[0085] Based on the parameters shown in Table 1, when the transition resistance is set to 0.01Ω, the reverse traveling wave spectrum of protection 1 at different fault distances can be obtained using the transmission line frequency variation parameters, any position not at the beginning or end of this line, fault at the beginning of this line, fault at the end of this line, fault at the beginning of the adjacent line, any position not at the beginning or end of the adjacent line, and the reverse traveling wave expression of the adjacent line. Figure 11 As shown in the diagram, the distances marked in the legend indicate the location of the fault point from the protection installation location. For example, 105km means the fault occurred 105km from the installation location of protection 1, which is an intra-zone fault. The adjacent line distance of 100km indicates the fault occurred on the downstream line in the positive direction of protection 1. l 3, which is 100km away from the installation location of protection device 5, is considered an out-of-zone fault. Figure 11 It is known that in the low-frequency band, the amplitude of the voltage reverse traveling wave line mode fault component does not decrease with the increase of fault distance; for example, the two curves for faults at the beginning and end of the line almost overlap. However, when the frequency exceeds 1000Hz, the voltage reverse traveling wave line mode fault component gradually decreases with the increase of fault distance, and the amplitude differences between the voltage reverse traveling wave line mode fault components under different fault distance conditions become more prominent. This is more conducive to constructing a staged criterion. At the same time, this characteristic becomes more obvious with further increases in frequency. Therefore, this invention patent adopts a 100kHz sampling rate commonly used in traveling wave protection, and selects high-frequency components based on this to construct the fault area detection criterion.

[0086] To further investigate the high-frequency component's ability to withstand transition resistance, ensuring that segment I can operate with 80% of the transition resistance on the main line but not during metallic faults at the beginning of adjacent lines, the following formula can be obtained (the parameters have the same meaning as above).

[0087]

[0088] Substituting the transmission line parameters into the above formula, we can obtain... Figure 12 .Depend on Figure 12 It can be seen that, when a suitable frequency is selected, under certain fault distance conditions, the transient resistance withstand capability of segment I can reach approximately 300Ω. Therefore, appropriate signal processing methods are needed to extract suitable high-frequency components.

[0089] Table 1 Parameters of Multi-Terminal Flexible DC Grid

[0090] Taking protection 7 as an example, operating conditions a and b are selected to test the performance of the proposed protection method.

[0091] Condition a: Protection 7 experiences a 300Ω high-resistance single-pole ground fault at 80% of the line length; Condition b: A metallic single-pole grounding fault occurs in the near-zone outside the protection zone of the protection system.

[0092] Based on operating conditions a and b, Figure 13 The positive voltage mutation, zero-mode voltage mutation, forward and reverse traveling wave fault components, and directional high-frequency components at protection point 7 are given.

[0093] According to the proposed protection method, the first step is to determine whether the protection is activated. Figure 13 It can be seen that when a fault occurs, the positive voltage surge at protection point 7 increases accordingly, with amplitudes exceeding 15kV. This indicates that the proposed protection start-up criterion can also achieve effective start-up with a transition resistance of 300Ω. Next, the second step is to determine the fault type. When a positive ground fault occurs, the fault type is determined by… Figure 13 The zero-mode voltage mutation at point 7 of the intermediate protection system shows that the zero-mode voltage mutations in both operating conditions a and b are trending negative. The accumulated value of these zero-mode voltage mutations allows for effective fault type identification. For example, the accumulated zero-mode voltage of R7 in operating condition a is -390.68kV, while in operating condition b it is -553.90kV, indicating a positive grounding fault, consistent with the actual fault. The third step is to determine the fault direction. Figure 13 From the voltage reverse and forward traveling wave mode fault components at protection point 7, it can be seen that, regardless of operating condition a or b, the ratio between the voltage reverse and forward traveling wave mode fault components at protection point 7 is much less than 2, indicating a forward fault, which is consistent with the actual settings. The fourth step is the fault area detection criterion for section I, based on... Figure 13 From the maximum directional high-frequency value at point 7 of the protection device, it can be determined that under operating condition a, the maximum directional high-frequency voltage value of protection device 7 is... DF The voltage is 4.037kV, while under operating condition b, the maximum directional high-frequency voltage of protection 7 is... DF The voltage is 1.22kV. Based on these values, it can be determined that the protection 7 in condition a meets the fault area detection criteria of stage I and belongs to the fault area; while the protection 7 in condition b does not meet the fault area detection criteria of stage I and needs to be verified by the fault area detection criteria of stage II.

[0094] For condition b, protections 6 and 5 are equivalent to faults within the zone, but protection 5 is a terminal fault. The downstream line fault protection that works in conjunction with protection 5 is protection 7. Therefore, Figure 14Given the directional high-frequency voltages of protections 5 and 6, the line-mode voltage abrupt changes of protections 5 and 7, and their first-order differentials under operating condition b. Figure 14 It can be seen that the maximum value of the directional high-frequency voltage of protection 5 is 1.20, which is less than the detection criterion for the first stage fault area. Therefore, the second stage fault area detection is also required.

[0095] Depend on Figure 14 Based on the maximum directional high-frequency voltage of protection 6, protection 6 meets the detection criteria for the first-stage fault area. Therefore, the hybrid high-voltage DC circuit breaker located at protection 6 will trip. Considering the first-stage operating time of 0.8 × 10⁻⁶, this indicates that protection 6 meets the detection criteria for the first-stage fault area. -3 The operating time of the hybrid high-voltage DC circuit breaker is 3×10 seconds. -3 Seconds, through 3.8 × 10 -3 A delay of seconds can be obtained Figure 14 The line-mode voltage abrupt changes shown in Protection 5 and Protection 7 reveal that the line-mode voltage abrupt change in Protection 5 exhibits a decreasing trend, while the line-mode voltage abrupt change in Protection 7 exhibits an increasing trend. This trend is further illustrated by their first-order difference. Using the first-order difference of the line-mode voltage abrupt changes and... Figure 15 The cross-correlation coefficients of the positive polarity step signals shown are calculated, yielding a cross-correlation coefficient of -0.9481 for protection 5 and 0.7705 for protection 7. Therefore, according to the fault zone detection criterion for stage II, protection 5 is determined to be an in-zone fault, and protection 7 is determined to be an out-of-zone fault. This is consistent with the actual settings.

[0096] This disclosure can be a system, method, and / or computer program product. A computer program product may include a computer-readable storage medium having computer-readable program instructions loaded thereon for causing a processor to implement various aspects of this disclosure.

[0097] Computer-readable storage media can be tangible devices capable of holding and storing instructions for use by an instruction execution device. Computer-readable storage media can be, for example—but not limited to—electrical storage devices, magnetic storage devices, optical storage devices, electromagnetic storage devices, semiconductor storage devices, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of computer-readable storage media include: portable computer disks, hard disks, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), static random access memory (SRAM), portable compact disc read-only memory (CD-ROM), digital multifunction disc (DVD), memory sticks, floppy disks, mechanical encoding devices, such as punch cards or recessed protrusions storing instructions thereon, and any suitable combination of the foregoing. The computer-readable storage media used herein are not to be construed as transient signals themselves, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through waveguides or other transmission media (e.g., light pulses through fiber optic cables), or electrical signals transmitted through wires.

[0098] The computer-readable program instructions described herein can be downloaded from computer-readable storage media to various computing / processing devices, or downloaded via a network, such as the Internet, local area network, wide area network, and / or wireless network, to an external computer or external storage device. The network may include copper transmission cables, fiber optic transmission, wireless transmission, routers, firewalls, switches, gateway computers, and / or edge servers. A network adapter card or network interface in each computing / processing device receives the computer-readable program instructions from the network and forwards them to the computer-readable storage media in the respective computing / processing device.

[0099] Computer program instructions used to perform the operations of this disclosure may be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-dependent instructions, microcode, firmware instructions, status setting data, or source code or object code written in any combination of one or more programming languages, including object-oriented programming languages ​​such as Smalltalk, C++, etc., and conventional procedural programming languages ​​such as the "C" language or similar programming languages. The computer-readable program instructions may execute entirely on the user's computer, partially on the user's computer, as a standalone software package, partially on the user's computer and partially on a remote computer, or entirely on a remote computer or server. In cases involving a remote computer, the remote computer may be connected to the user's computer via any type of network—including a local area network (LAN) or a wide area network (WAN)—or may be connected to an external computer (e.g., via the Internet using an Internet service provider). In some embodiments, electronic circuitry, such as programmable logic circuitry, field-programmable gate arrays (FPGAs), or programmable logic arrays (PLAs), is personalized by utilizing the status information of the computer-readable program instructions to implement various aspects of this disclosure.

[0100] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A boundaryless protection method for multi-terminal flexible DC power grid transmission lines, characterized in that, include: S1: Analyze the amplitude-frequency difference of faults inside and outside the boundaryless transmission line area of ​​the multi-terminal flexible DC grid, and obtain the characteristic frequency range of the line-mode reverse voltage traveling wave fault component inside the area; S2: Real-time acquisition of positive voltage, negative voltage, positive current, and negative current at the installation location of the multi-terminal flexible DC power grid boundaryless transmission line protection for single-ended quantity protection; S3: Calculate the voltage change based on the positive and negative voltages, use the voltage change to detect the protection start-up, and record the disturbance time value after the protection starts. S4: Set the disturbance time interval based on the disturbance moment, calculate the accumulated zero-mode voltage for fault detection; S5: Based on the real-time collected positive voltage, negative voltage, positive current, and negative current, calculate the line-mode forward voltage traveling wave fault component and the line-mode reverse voltage traveling wave fault component within the disturbance time interval. Determine the fault direction by comparing the absolute value of the ratio of the line-mode forward voltage traveling wave fault component to the line-mode reverse voltage traveling wave fault component with a direction threshold, and obtain the fault direction index. D r ; S6: Based on the pre-acquired characteristic frequency range of the line-mode reverse voltage traveling wave fault component within the region, perform wavelet packet transform on the line-mode reverse voltage traveling wave fault component to obtain the maximum value of the directional high-frequency voltage. DF ,like DF If the voltage exceeds the first high-frequency voltage threshold, it is determined to be an intra-zone fault; if DF If the voltage is less than or equal to the second high-frequency voltage threshold, it is judged as an external fault; otherwise, the line-mode voltage change is obtained after a delay and the normalized line-mode voltage value is obtained. When the cross-correlation coefficient between the normalized line-mode voltage value and the set positive polarity step signal is less than 0, it is judged as an internal fault; otherwise, it is an external fault.

2. The method for boundaryless protection of multi-terminal flexible DC power grid transmission lines according to claim 1, characterized in that: In S1, the characteristic frequency range of the reverse voltage traveling wave fault component in the fault line mode within the region is obtained in advance using the reverse traveling wave analysis method. For any location not at the beginning or end of this line, a fault at the end of this line, a fault at the beginning of this line, and any location not at the beginning or end of an adjacent line, establish an expression for the reverse voltage traveling wave fault component and solve for its line-mode reverse voltage traveling wave fault component. Plot the amplitude-frequency curves of the reverse voltage traveling wave fault component of the line mode at different fault locations to obtain the characteristic frequency range of the reverse voltage traveling wave fault component of the fault line mode within the transmission line area.

3. The boundaryless protection method for multi-terminal flexible DC power grid transmission lines according to claim 1, characterized in that: In S3, based on the collected positive voltage... U p ( t ), negative voltage U n ( t Average rated voltage of the positive pole of a multi-terminal flexible DC power grid U rp Average rated voltage of the negative pole of a multi-terminal flexible DC power grid U rn Calculate the corresponding positive fault sudden voltage respectively. U pm ( t ) and negative electrode fault sudden voltage U nm ( t ): In the formula, t Time is measured in seconds, and its symbol is s. When detected U pm ( t )or U nm ( t (Greater than the rated voltage of the positive terminal of a multi-terminal flexible DC power grid) U ra of sx If the value is multiple times the threshold, it is determined that a disturbance has occurred in the multi-terminal flexible DC power grid, the protection system is activated, and the value at that moment is recorded. t fs As the time value of the disturbance.

4. The boundaryless protection method for multi-terminal flexible DC power grid transmission lines according to claim 1, characterized in that: Determined based on the principle that even the largest possible single-pole grounding fault transition resistance in a multi-terminal flexible DC power grid can ensure normal startup. sx The value of .

5. The boundaryless protection method for multi-terminal flexible DC power grid transmission lines according to claim 1, characterized in that: In S4, the accumulated zero-mode voltage is calculated according to the following formula: when When a positive ground fault is detected, it is determined that a ground fault has occurred; when If the fault occurs, it is determined that a negative grounding fault has occurred; otherwise, it is determined that a double grounding fault has occurred. in, For the time of disturbance, This refers to the disturbance time interval set based on the disturbance time. , These are the pre-extension time and the post-extension time, respectively. The zero-mode voltage is calculated over the disturbance time interval. This is the threshold for zero-mode voltage accumulation.

6. The boundaryless protection method for multi-terminal flexible DC power grid transmission lines according to claim 5, characterized in that: The zero-mode voltage accumulation threshold U jx The tuning calculation is performed according to the following formula. In the formula, k jx For fault selection, the reliability coefficient is used. f s The signal sampling frequency, T w For signal time window, U ra This is the rated voltage of the positive terminal of the power grid.

7. The boundaryless protection method for multi-terminal flexible DC power grid transmission lines according to claim 6, characterized in that: Select Second; seconds; in f s When the frequency is 100kHz, select k jx It is 0.

01.

8. The boundaryless protection method for multi-terminal flexible DC power grid transmission lines according to claim 1, characterized in that: In S5, the fault direction is determined based on the following criteria, and the fault direction index Dr is obtained: Forward fault: ; Reverse fault: ; in, , These represent the forward voltage traveling wave fault component and the reverse voltage traveling wave fault component of the line mode within the disturbance time interval, respectively. The set direction threshold.

9. The boundaryless protection method for multi-terminal flexible DC power grid transmission lines according to claim 8, characterized in that: Linear mode forward voltage traveling wave fault component and line mode reverse voltage traveling wave fault component Calculate using the following formula: In the formula, and These represent the fault components of line-mode voltage and line-mode current during the disturbance time interval, respectively. This represents the mode impedance of the transmission line.

10. The boundaryless protection method for multi-terminal flexible DC power grid transmission lines according to claim 1, characterized in that: In S6, the fault component Δ of the line-mode reverse voltage traveling wave is... u B Wavelet packet decomposition is performed. Based on the characteristic frequency range of the reverse voltage traveling wave fault component of the fault line mode within the transmission line area, the wavelet packet is selected according to the frequency range from small to large. cs Layer wavelet packet reconstruction coefficients are used to obtain the high-frequency traveling wave fault component Δ. u HB Then, the maximum value of the high-frequency voltage in the direction is calculated according to the following formula. DF : Where, Δ u HDB For directional high-frequency voltage, DF The maximum value of the high-frequency voltage in the direction after the disturbance moment. cs The selection is made by combining the sampling frequency of the traveling wave protection signal and the characteristic frequency range of the reverse voltage traveling wave fault component of the fault line mode in the transmission line area obtained in advance.

11. The method for boundaryless protection of multi-terminal flexible DC power grid transmission lines according to claim 1 or 10, characterized in that: When the fault is a single-pole fault: The first high-frequency voltage threshold should be greater than that of a near-field metallic unipolar fault outside the fault zone. DF 1.1 times the value; The range of the second high-frequency voltage threshold is selected based on the operating condition of the DC circuit breaker's energy-consuming branch when it cannot start under a single-pole fault. When the fault is a bipolar fault: The first high-frequency voltage threshold value should be greater than that of a near-field metallic bipolar fault outside the fault zone. DF 1.1 times the value; The range of the second high-frequency voltage threshold is selected based on the operating condition of the DC circuit breaker's energy-consuming branch when it cannot start under bipolar fault.

12. The boundaryless protection method for multi-terminal flexible DC power grid transmission lines according to claim 11, characterized in that: In S6, when the direction is towards the maximum value of the high-frequency voltage DF When the voltage is greater than the second high-frequency voltage threshold but less than the first high-frequency voltage threshold. Obtain the line-mode voltage change Δ at the protection installation location. U 1. Obtain the normalized line-mode voltage Δ GU 1. Find Δ GU 1 and positive polarity step signal J ref ( t Cross-correlation coefficients between ) CF ,like CF If the value is less than 0, it is determined to be a fault within the area; if CF If the value is greater than 0, it is determined to be an external fault.

13. A boundaryless protection system for multi-terminal flexible DC power grid transmission lines utilizing the method described in any one of claims 1-12, characterized in that, The system includes: The reverse voltage traveling wave fault component analysis module analyzes the amplitude-frequency difference of faults inside and outside the boundaryless transmission line of the multi-terminal flexible DC grid based on the system topology and transmission line parameters, and obtains the characteristic frequency range of the reverse voltage traveling wave fault component of the line mode inside the boundary. The single-ended quantity information acquisition module is used to acquire the positive voltage, negative voltage, positive current, and negative current at the installation location of the multi-terminal flexible DC power grid boundaryless transmission line protection for single-ended quantity protection. The protection start detection module calculates the voltage change based on the positive and negative voltages, uses the voltage change to detect the protection start, and records the disturbance time value after the protection starts. The fault polarity detection module calculates and accumulates the zero-mode voltage to detect the fault polarity based on the disturbance time interval set at the disturbance time. The fault direction determination module determines the fault direction by comparing the absolute value of the ratio of the forward voltage traveling wave to the reverse voltage traveling wave of the line mode with a direction threshold, and obtains the fault direction index to determine the fault direction. The fault area detection module, based on the pre-acquired characteristic frequency range of the line-mode reverse voltage traveling wave fault component within the area, performs wavelet packet transform on the line-mode reverse voltage traveling wave, and combines this with the traveling wave protection signal sampling frequency to obtain the maximum value of the high-frequency voltage in the direction. DF ,like DF If the voltage exceeds the first high-frequency voltage threshold, it is determined to be an intra-zone fault; if DF If the voltage is less than or equal to the second high-frequency voltage threshold, it is judged as an external fault; otherwise, the line-mode voltage change is obtained after a delay and the normalized line-mode voltage value is obtained. When the cross-correlation coefficient between the normalized line-mode voltage value and the set positive polarity step signal is less than 0, it is judged as an internal fault; otherwise, it is an external fault.

14. A computer device, comprising a memory, a processor, and a computer program stored in the memory; characterized in that: The processor executes the computer program to implement the steps of the method according to any one of claims 1-12.

15. A computer-readable storage medium having a computer program stored thereon, characterized in that, When executed by a processor, the computer program implements the steps of the method according to any one of claims 1-12.

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