Single-ended setting-free protection method for multi-terminal flexible direct current power grid based on first line wave similarity

Abstract

This invention discloses a method for single-terminal quantity-free setting protection of multi-terminal flexible DC power grids based on first-traveling wave similarity, comprising: S1, decoupling the voltage and current at the positive and negative pole protection points of the local terminal; S2, acquiring the first-traveling wave of the line mode and the first-traveling wave of the ground mode, and acquiring the measured value of the first-traveling wave of the fault pole; S3, determining the intra-regional similarity and inter-regional similarity based on the analytical value and the measured value of the first-traveling wave; S4, when the intra-regional similarity is greater than the inter-regional similarity for 5 consecutive times, determining the fault type as an intra-regional fault; otherwise, repeating steps S1 to S4 until the number of repetitions of steps S1 to S4 is 5, wherein the intra-regional similarity S internal Not five consecutive times greater than the out-of-region similarity S external In the case of an out-of-area fault, the fault type is determined to be an out-of-area fault; S5, when the fault type is an in-area fault, fault selection is performed based on the fault selection criterion. This application can identify out-of-area faults by comparing the magnitude of in-area similarity and out-of-area similarity, without the need to adjust the set value.

Description

Technical Field

[0001] This invention belongs to the field of power system relay protection, specifically involving a method for single-terminal quantity protection without setting in a multi-terminal flexible DC power grid based on the similarity of the first traveling wave. Background Technology

[0002] Flexible DC transmission technology based on modular multilevel converters (MMC) has demonstrated significant advantages in passive system power supply, flexible interconnection of regional power grids, and the development and utilization of large-scale deep-sea wind power due to its low harmonic content, high controllability, and absence of commutation failure. To further improve the reliability and flexibility of flexible DC transmission, constructing a multi-terminal flexible DC grid based on MMC is an important direction for the future application of flexible DC transmission technology. my country's ±500kV Zhangbei Flexible DC Project, which has already been completed and put into operation, is the world's first flexible DC grid project and a truly network-like DC grid.

[0003] To minimize the outage area, DC power grids require selective protection. For flexible DC power grids, current-limiting reactors are typically installed at line outlets to suppress the rate of rise of fault current, providing a natural boundary for protection selectivity. Compared to point-to-point DC transmission systems, the transient characteristics of MMC-MTDC power grids, which are powered by multiple sources and have multiple receiving points, are more complex after a fault. Because flexible DC power grids are low-inertia systems with low network damping, fault currents rise rapidly and have large amplitudes. Therefore, flexible DC power grids require high protection speed; for example, the Zhangbei four-terminal flexible DC power grid requires protection to trip within 3ms after a fault. Therefore, to prevent further escalation of the accident, fault isolation must be rapid and reliable, which places higher demands on the protection principles applicable to multi-terminal flexible DC power grids.

[0004] Currently, the main problems with protection schemes for flexible DC power grids at home and abroad are as follows: (1) In existing research, protection criteria generally require setting values, and the contradictions between reliability, sensitivity, selectivity and speed need to be coordinated through setting values. Once the setting value is not set properly, the protection performance will be seriously affected. There have been accidents in existing DC projects where protection maloperation was caused by setting value problems. (2) Existing protection schemes are easily affected by transition resistance. When there is a high resistance fault, it may cause protection misjudgment. It is necessary to study protection principles that are not affected by transition resistance. (3) The low damping characteristics of flexible DC power grids make the speed of fault propagation in flexible DC power grids extremely fast. Therefore, multi-terminal flexible DC power grids have higher requirements for the speed of protection. It is necessary to study protection based on single-terminal quantities to further improve the speed of protection.

[0005] In summary, the research on single-terminal quantity-free protection applicable to multi-terminal flexible DC power grids, which can reliably identify faults inside and outside the protection zone, is of great significance for improving the safety and reliability of flexible DC power grids. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for setting-free protection of single-terminal quantities in multi-terminal flexible DC power grids based on the similarity of the first traveling wave.

[0007] The objective of this invention is achieved through the following technical solution: a single-terminal quantity-free setting protection method for multi-terminal flexible DC power grids based on first-traveling wave similarity. This method can be executed by one or more processors, including: S1, after protection activation, the one or more processors decouple the voltage and current at the positive and negative poles of the protection terminal based on a phase-mode transformation matrix; S2, the one or more processors acquire the line-mode first-traveling wave and the ground-mode first-traveling wave in the mode domain, and acquire the measured value of the first-traveling wave at the fault pole based on the phase-mode inverse transformation; S3, the one or more processors normalize the measured value of the first-traveling wave, and determine the intra-regional similarity S based on the analytical value of the first-traveling wave and the normalized measured value of the first-traveling wave. internal Similarity S between regions external S4, the similarity S within the region internal Five consecutive times the similarity S outside the region is greater than the stated similarity. external In the event of a fault, the one or more processors determine the fault type as an intra-region fault; otherwise, steps S1 to S4 are repeated until the number of repetitions of steps S1 to S4 is 5, wherein the intra-region similarity S internal Not five consecutive times greater than the out-of-region similarity S external In the case of an external fault, the one or more processors determine the fault type as an external fault; S5, in the case of an internal fault, the one or more processors perform fault polarity selection based on the fault polarity selection criterion composed of the zero-mode first traveling wave, so as to further classify the internal fault into positive ground fault, negative ground fault and bipolar short circuit fault.

[0008] Preferably, steps S1 to S3 are repeated five times, wherein the similarity S within the region is... internal Five consecutive times the similarity S outside the region is greater than the stated similarity. external In the event of a fault, the one or more processors will determine the fault type as an in-zone fault; otherwise, the one or more processors will determine the fault type as an out-of-zone fault.

[0009] Preferably, the first traveling wave analytical value includes the first traveling wave analytical value transmitted to the local protection point when a fault occurs on the line side of the current-limiting reactor at the opposite end of the line. The frequency domain expression of the first traveling wave analytical value is: Where A1(s) is the line-mode traveling wave transfer function; τ1 is the line-mode traveling wave propagation time; m1 is the number of zeros and poles of the line-mode transfer function; q 1i p is the residue of the line-mode traveling wave transfer function; 1is represents the poles of the line-mode traveling wave transfer function, and s is the complex frequency.

[0010] Preferably, the first traveling wave analytical value also includes a second first traveling wave analytical value transmitted to the local protection point when a fault occurs on the bus side of the current-limiting reactor at the opposite end of the line. The frequency domain expression of the second first traveling wave analytical value is: Among them, B M1 (s) represents the first traveling wave transmitted to the local protection point when a fault occurs on the bus side of the current-limiting reactor, Z. c1 (s) represents the wave impedance; L represents the current-limiting reactor.

[0011] Preferably, the one or more processors are capable of processing the frequency domain expression of the first traveling wave analytical value to obtain the time domain analytical expression: Among them, a i b i c i Δt represents the recursive convolution coefficients of the line-mode transfer function, and Δt represents the sampling interval.

[0012] Preferably, the one or more processors are capable of processing the frequency domain expression of the first traveling wave analytical value two to obtain the time domain analytical expression:

[0013] Preferably, the fault selection criterion is: Among them, b M0set k is the threshold value of the first traveling wave of a zero-mode fault. set b is the reliability coefficient of the pole selection criterion. M0 This is the first traveling wave for a zero-mode fault.

[0014] Preferably, the one or more processors are capable of processing the b M0set Set to 300kV, and set the k set Set to 0.3 。

[0015] Preferably, the intra-regional similarity S internal Similarity S between regions external It can be determined using the following formula: In the formula: N is the number of sampling points; X i Y is the i-th sampling point; j Let j be the j-th sampling point.

[0016] Preferably, the intra-region similarity is used to characterize the similarity between the measured first traveling wave at this end and the analytical value of the first traveling wave transmitted from the line-side fault of the current-limiting reactor at the opposite end of the line to the protection point at this end, and the extra-region similarity is used to characterize the similarity between the measured first traveling wave at this end and the analytical value of the first traveling wave transmitted from the bus-side fault of the current-limiting reactor at the opposite end of the line to the protection point at this end.

[0017] The present invention has the following advantages: the present application can identify faults inside and outside the area by comparing the similarity between the area and the area, without the need to adjust the setting value, and the normalization process can effectively eliminate the influence of the transition resistance, so that the present application will not be affected by the transition resistance. The present application only uses single-ended quantity to achieve the protection speed can meet the requirements of multi-terminal flexible DC power grid. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the fault components of the present invention;

[0019] Figure 2 The frequency domain equivalent plot of the Marti model;

[0020] Figure 3 Peterson equivalent diagram;

[0021] Figure 4 This is a schematic diagram of a fault on the line side of a current-limiting reactor.

[0022] Figure 5 This is a schematic diagram of a fault on the bus side of a current-limiting reactor.

[0023] Figure 6 This is a flowchart illustrating the single-ended quantity protection method for multi-terminal flexible DC power grids based on the similarity of the first traveling wave.

[0024] Figure 7 This is a schematic diagram of the fault identification results within the area;

[0025] Figure 8 This is a schematic diagram of the fault identification results outside the area. Detailed Implementation

[0026] The following is in conjunction with the appendix Figure 1 To be continued Figure 8 The present invention will be further described below, but the scope of protection of the present invention is not limited to the following:

[0027] 1. Analysis of the first line of wave characteristics during a fault

[0028] (1) Characteristics of the first wave amplitude in the fault

[0029] Taking a positive electrode circuit fault as an example for analysis, the circuit fault component diagram is as follows: Figure 1 As shown, Figure 1 In this context, P is the positive terminal and N is the negative terminal. The boundary conditions for the fault components can be obtained as follows:

[0030]

[0031] In equation (1), U fp (s), U fn (s) represent the positive and negative voltages to ground at the fault point, respectively; U f For memory voltage; Rf For transition resistance; I fp (s), I fn (s) represent the positive and negative currents to ground at the fault point, respectively.

[0032] Since there is a coupling relationship between the electrical quantities of the positive and negative poles, they need to be decoupled first, and the pole electrical quantities need to be converted into independent modal electrical quantities. The phase-mode transformation formula is as follows:

[0033]

[0034] In equation (2): x1 and x0 are the electrical quantities of the line mode and the ground mode, respectively; x p x n These are the positive and negative electrical quantities, respectively.

[0035] After decoupling, the fault point modal network can be represented by the following equation:

[0036]

[0037] In equation (3): U f1 (s), U f0 (s) represent the line-mode and ground-mode voltages at the fault point, respectively; I f1 (s), I f0 (s) represent the line-mode and ground-mode currents at the fault point, respectively; Z c1 (s), Z c0 (s) represent the impedances of the line mode and the ground mode, respectively.

[0038] By combining equations (1)-(3), the expression for the amplitude of the first traveling wave in a single-pole grounding fault can be obtained as follows:

[0039]

[0040] Similarly, the amplitude of the first wave during a bipolar short-circuit fault can be obtained as follows:

[0041]

[0042] As can be seen from equations (4) and (5), regardless of whether it is a single-pole ground fault or a double-pole short-circuit fault, the transition resistance only affects the amplitude of the first traveling wave of the fault, and does not affect its waveform. Therefore, in order to improve the protection's ability to withstand the transition resistance, the first traveling wave of the fault can be normalized and processed into a first traveling wave with an amplitude of 1 for subsequent calculations, thereby effectively eliminating the influence of the transition resistance on the first traveling wave.

[0043] (2) Fault first traveling wave propagation characteristics

[0044] When a fault occurs inside or outside the zone, the first traveling wave will pass through the transmission line and the current-limiting reactor during transmission. The impact of these two factors on the transmission process of the first traveling wave will be analyzed below.

[0045] a. Attenuation and distortion of the initial traveling wave by the line

[0046] The line model adopts the Marti model, which considers the distribution characteristics and frequency variation characteristics of line parameters. Its frequency domain equivalent is as follows: Figure 2 As shown. If only the electrical quantities at this terminal are used, B M (s) and B N (s) can be obtained from the following formulas:

[0047]

[0048] In equation (6), U M (s), U N (s) represent the voltages across M and N, respectively; I M (s), I N (s) represent the currents at terminals M and N, respectively; B M (s), B N (s) represent the anti-traveling waves at M and N ends, respectively; Z c (s) represents the line surge impedance.

[0049] If only the electrical quantities at the opposite end are used, B M (s) and B N (s) can also be obtained from the following formulas:

[0050]

[0051] In equation (7): F M (s) and F N A(s) represents the traveling waves at M and N, respectively; A(s) is the transfer function.

[0052] The wave impedance and transfer function in the Marti model can be expressed as follows:

[0053]

[0054] In equation (8): q0 is a free term, q i For residue; p i λ is the pole; k is the wave impedance residue and the number of poles; m is the transfer function residue and the number of poles; τ is the fastest transmission time of the traveling wave on the entire transmission line, τ = l / v, where l is the distance from the fault location to the protection measurement point, v is the maximum wave velocity of the traveling wave within the considered frequency range; s is the complex frequency.

[0055] As can be seen from the above analysis, the arrival time of the first traveling wave at the protection measurement point mainly depends on the fault location. The farther the fault distance, the longer the arrival time of the first traveling wave. The summation term in the transfer function represents the attenuation and distortion of the first traveling wave transmission. Since this study needs to normalize the first traveling wave, the effect of the first traveling wave attenuation is not significant. The main focus is on the distortion of the first traveling wave. Generally, the farther the fault location, the longer the transmission distance of the first traveling wave, and the more severe the distortion.

[0056] b. Refraction of the first traveling wave by the current-limiting reactor

[0057] When an external fault occurs, the initial traveling wave of the fault will be refracted after passing through the current-limiting reactor. It is necessary to analyze the refraction effect of the current-limiting reactor on the initial traveling wave. Taking a fault on the busside of the current-limiting reactor as an example, its Peterson equivalent diagram is as follows: Figure 3 As shown. The traveling wave at point N after refraction by the current-limiting reactor is:

[0058]

[0059] In equation (9): 2U f (s) represents the Peterson equivalent voltage source; L represents the current-limiting reactor.

[0060] After obtaining the traveling wave at point N, the first traveling wave reaching point M can be obtained after transmission through the line. As can be seen from the formula, the refraction of the current-limiting reactor further aggravates the distortion of the first traveling wave, resulting in a significant difference in the waveforms of the first traveling waves on both sides of the current-limiting reactor. It should be noted that due to the coupling between lines, the calculations in the above two sections should also be performed in the mode domain, and finally transformed to the phase domain.

[0061] In summary, the analysis above shows that the initial traveling wave waveforms transmitted to the opposite protection station when faults occur on both sides (line side and bus side) of the current-limiting reactor differ significantly. Furthermore, the initial traveling wave waveform for faults within the protection zone is more similar to that of a fault on the line side of the current-limiting reactor, while the initial traveling wave waveform for faults outside the protection zone is more similar to that of a fault on the bus side of the current-limiting reactor. Therefore, if the time-domain representation of the initial traveling wave transmitted to the opposite protection station when faults occur on both sides of the current-limiting reactor can be obtained, then when an actual fault occurs, comparing the measured initial traveling wave at the protection station with the aforementioned waveforms can effectively distinguish between faults within and outside the protection zone.

[0062] 2. Analytical Calculation of the First Traveling Wave of the Fault

[0063] Taking a positive pole fault as an example, we analyze the time-domain analytical expression of the first traveling wave transmitted to the local protection when the faults occur on the line side and the bus side of the opposite current-limiting reactor.

[0064] (1) Fault on the line side of the current-limiting reactor

[0065] A schematic diagram of a fault on the line side of a current-limiting reactor is shown below. Figure 4As shown, the analysis is conducted using a line-mode fault as an example. Since the line parameters are frequency-varying, the analysis is first performed in the frequency domain. Because the amplitude of the first traveling wave needs to be normalized, for a positive fault, the amplitude of the first traveling wave can be directly taken as -1. After transmission through the line, the frequency domain expression of the fault's first traveling wave measured at the first-end protection is:

[0066]

[0067] In equation (10): A1(s) is the line-mode traveling wave transfer function; τ1 is the line-mode traveling wave propagation time; m1 is the number of zeros and poles of the line-mode transfer function; q 1i p is the residue of the line-mode traveling wave transfer function; 1i These are the poles of the line-mode traveling wave transfer function.

[0068] Applying the inverse Laplace transform and recursive convolution transform to the above equation, the time-domain analysis of the first traveling wave of the line mode at the M-end protection point can be obtained as follows:

[0069]

[0070] In the formula: a i b i c i q represents the recursive convolution coefficients of the line-mode transfer function, Δt is the sampling interval, f1(t-μ) is function 1; f2(μ) is function 2; t is time; μ is a variable; q i For residue; p i b is the extreme point; M1 (t-Δt) represents the historical value of the previous point.

[0071] Similarly, the time-domain analytical expression of the zero-mode first traveling wave can be obtained, and then the first traveling wave at point M of the positive protection when the current-limiting reactor experiences a line-side fault can be obtained through phase-mode inverse transformation.

[0072] 2.2 Fault on the bus side of the current-limiting reactor

[0073] A schematic diagram of a fault on the line side of a current-limiting reactor is shown below. Figure 5 As shown, the frequency domain expression of the first traveling wave at the M-end protection point in this scenario is:

[0074]

[0075] Similarly, by performing inverse Laplace transform and recursive convolution transform on the above equation, the time-domain analysis of the first traveling wave of the line mode at the M-end protection point can be obtained as follows:

[0076]

[0077] Similarly, the time-domain analytical expression of the zero-mode first traveling wave can be obtained, and then the first traveling wave at point M of the positive protection when the current-limiting reactor experiences a fault on the bus side can be obtained through phase-mode inverse transformation.

[0078] 3. Flexible DC power grid protection scheme without setting

[0079] (1) Fault start

[0080] To avoid frequent actions in the fault detection process, it is necessary to design a fault initiation criterion. Currently, research on fault initiation criteria is relatively mature, with typical examples in engineering being voltage change rate criteria and current change rate criteria. This invention utilizes the characteristics of voltage change rate, and the fault initiation criterion is as follows:

[0081]

[0082] In the formula: u AB This is the measured voltage at the protection point on this end; u set The threshold for initiating the decision criteria.

[0083] (2) Fault identification

[0084] First, the similarity S within the region is... internal Similarity S between regions external Define:

[0085] S internal The similarity between the measured first traveling wave at this end and the analytical value of the first traveling wave transmitted from the current-limiting reactor at the line side of the line to the protection point at this end;

[0086] S external The similarity between the measured first traveling wave at this end and the analytical value of the first traveling wave transmitted from the current-limiting reactor bus side fault at the opposite end of the line to the protection point at this end.

[0087] The similarity is calculated using Pearson similarity, and the formula is as follows:

[0088]

[0089] In equation (15): X represents the measured value of the first traveling wave; Y represents the resolved value of the first traveling wave; X i For the i-th sampling point, X j Y is the j-th sampling point; i For the i-th sampling point, Y j Let N be the j-th sampling point, and N be the number of sampling points.

[0090] When the similarity within the protection zone is greater than the similarity outside the zone, the fault is determined to be within the zone; otherwise, it is determined to be outside the zone. The protection criterion is expressed as follows:

[0091] S internal >S external (16)

[0092] To avoid malfunctions caused by interference such as lightning strikes, the system stipulates that if the above criteria are met five consecutive times, the fault is determined to be within the protection zone, and the protection operates; otherwise, the protection does not operate. The proposed solution only needs to compare the similarity between the faults inside and outside the protection zone to identify faults within and outside the zone, eliminating the need to set thresholds and thus achieving protection without adjustment.

[0093] (3) Fault selection

[0094] For positive ground faults, the zero-mode first traveling wave exhibits a rapid drop trend; for negative ground faults, the zero-mode first traveling wave exhibits a rapid rise trend; for bipolar short-circuit faults, the zero-mode first traveling wave is nonexistent (theoretically, it is 0). Based on these differences, the following fault polarity selection criterion is proposed:

[0095]

[0096] In equation (17): b M0set k is the threshold value of the first traveling wave of a zero-mode fault. set The reliability coefficient is used for pole selection criterion. Since the amplitude of the first wave during a fault decreases as the transition resistance increases, b is taken as the value for a high-resistance fault (500Ω). M0set The amplitude is 300kV as the minimum value, due to the bipolar short-circuit fault. M0 It is very small, almost zero, so considering a certain margin and the speed of fault selection, k set Set it to 0.3.

[0097] (4) Overall process

[0098] Figure 6 A flowchart of a single-terminal quantity-free protection method for multi-terminal flexible DC power grids based on first traveling wave similarity provided by the present invention includes the following steps:

[0099] S1, after the protection is started, the voltage and current at the positive and negative poles of this terminal are decoupled based on the phase mode transformation matrix;

[0100] S2, in the mode domain, obtain the first traveling wave of the line mode and the first traveling wave of the ground mode, and obtain the measured value of the first traveling wave of the fault pole based on the phase mode inverse transformation;

[0101] S3, after normalizing the measured values ​​of the first traveling wave, the intra-regional similarity S is determined based on the analytical values ​​of the first traveling wave and the normalized measured values ​​of the first traveling wave. internal Similarity S between regions external ;

[0102] S4, similarity S within the region internal Five consecutive times the similarity S outside the region is greater than the similarity S outside the region. external In the case of an intra-regional fault, the fault type is determined to be an intra-regional fault; otherwise, steps S1 to S4 are repeated until the number of repetitions of steps S1 to S4 is 5. The intra-regional similarity S...internal Not five consecutive times greater than the out-of-region similarity S external In such cases, the fault type will be classified as an external fault.

[0103] S5, when the fault type is an intra-zone fault, the fault pole selection is performed based on the fault pole selection criterion composed of the zero-mode first traveling wave, so as to further classify the intra-zone fault into positive ground fault, negative ground fault and bipolar short circuit fault.

[0104] Figure 7 This is a schematic diagram of the fault identification results within a designated area provided in an embodiment of the present invention. Figure 7 It can be seen that the measured waveform of the first traveling wave at this end during an intra-zone fault is quite similar to the waveform of the first traveling wave transmitted from the line side fault of the current-limiting reactor at the opposite end to the protection point at this end, but differs significantly from the waveform of the first traveling wave transmitted from the bus side fault of the current-limiting reactor at the opposite end to the protection point at this end. Fault identification begins at 0.6ms, and the intra-zone similarity at the subsequent five consecutive judgment points is greater than the inter-zone similarity, satisfying the protection action criterion. The protection operates at 0.8ms, demonstrating that the proposed scheme can quickly and reliably identify intra-zone faults.

[0105] Figure 8 This is a schematic diagram illustrating the results of fault identification outside the designated area provided in an embodiment of the present invention. Figure 8 It can be seen that the measured waveform of the first traveling wave at this end during an external fault is quite similar to the waveform of the first traveling wave transmitted from the bus side of the current-limiting reactor at the opposite end to the protection point at this end, but differs significantly from the waveform of the first traveling wave transmitted from the line side of the current-limiting reactor at the opposite end to the protection point at this end. Fault identification begins at 1.0ms, and the similarity within the fault zone is less than the similarity outside the fault zone for the next five consecutive judgment points. At 1.2ms, it is determined to be an external fault, and the protection does not operate. The proposed scheme can quickly and reliably identify external faults.

[0106] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for single-ended quantity protection without setting in a multi-terminal flexible DC power grid based on first-traveling wave similarity, characterized in that: The method can be executed by one or more processors, including: S1, After the protection is activated, the one or more processors decouple the voltage and current at the positive and negative protection points of this terminal based on the phase mode transformation matrix; S2, the one or more processors acquire the first traveling wave of the line mode and the first traveling wave of the ground mode in the mode domain, and acquire the measured value of the first traveling wave of the fault pole based on the phase mode inverse transformation; S3, the one or more processors normalize the measured values ​​of the first traveling wave, and determine the intra-region similarity based on the resolved values ​​of the first traveling wave and the normalized measured values ​​of the first traveling wave. S internal Similarity with areas outside the region S external ; S4, similarity within the region S internal Five consecutive times the similarity to the outside region is greater than the stated similarity. S external In the event of a fault, the one or more processors determine the fault type as an intra-regional fault; otherwise, steps S1 to S4 are repeated until the number of repetitions of steps S1 to S4 is 5. The intra-regional similarity... S internal Not five consecutive times greater than the out-of-region similarity S external In this case, the one or more processors will determine the fault type as an out-of-area fault; S5, when the fault occurrence type is an intra-zone fault, the one or more processors perform fault polarity selection based on the fault polarity selection criterion composed of the zero-mode first traveling wave, so as to further divide the intra-zone fault into positive ground fault, negative ground fault and bipolar short circuit fault. The fault polarity selection criterion is as follows: ,in, b M0set The threshold value for the first traveling wave of a zero-mode fault. k set The reliability coefficient is the selection criterion. b M0 This is the first traveling wave for a zero-mode fault.

2. The method for single-terminal quantity protection without setting in a multi-terminal flexible DC power grid according to claim 1, characterized in that, Repeat steps S1 to S3 five times, wherein the similarity within the region is... S internal Five consecutive times the similarity to the outside region is greater than the stated similarity. S external In the event of a fault, the one or more processors will determine the fault type as an in-zone fault; otherwise, the one or more processors will determine the fault type as an out-of-zone fault.

3. The method for single-terminal quantity protection without setting in a multi-terminal flexible DC power grid according to claim 1, characterized in that, The first traveling wave analytical value includes the first traveling wave analytical value transmitted to the local protection point when a fault occurs on the line side of the current-limiting reactor at the opposite end of the line. The frequency domain expression of the first traveling wave analytical value is: ,in, B M1 ( s This is the first traveling wave transmitted to the local protection unit when a fault occurs on the bus side of the current-limiting reactor; A 1( s ) is the transfer function of the line-mode traveling wave; τ 1 represents the propagation time of the linear traveling wave; m 1 represents the number of zeros and poles in the line-mode transfer function; q 1i The residue of the line-mode traveling wave transfer function; p 1i These are the poles of the line-mode traveling wave transfer function; s It is a complex frequency.

4. The method for single-terminal quantity protection without setting in a multi-terminal flexible DC power grid according to claim 1, characterized in that, The first traveling wave analytical value also includes the second first traveling wave analytical value transmitted to the local protection point when a fault occurs on the bus side of the current-limiting reactor at the opposite end of the line. The frequency domain expression of the second first traveling wave analytical value is: ,in, B M1 ( s This is the first traveling wave transmitted to the local protection unit when a fault occurs on the bus side of the current-limiting reactor; Z c1 ( s () represents wave impedance; A 1( s ) is the transfer function of the line-mode traveling wave; τ 1 represents the propagation time of the linear traveling wave; m 1 represents the number of zeros and poles in the line-mode transfer function; q 1i The residue of the line-mode traveling wave transfer function; p 1i These are the poles of the line-mode traveling wave transfer function; s It is a complex frequency; L It is a current-limiting reactor.

5. The method for single-terminal quantity protection without setting in a multi-terminal flexible DC power grid according to claim 3, characterized in that, The one or more processors can process the frequency domain expression of the first traveling wave analytical value to obtain the time domain analytical expression: ,in, a i , b i , c i For the recursive convolution coefficients of the line-mode transfer function, The sampling interval is denoted as .

6. The method for single-terminal quantity protection without setting in a multi-terminal flexible DC power grid according to claim 4, characterized in that, The one or more processors can process the frequency domain expression of the first traveling wave analytical value two to obtain the time domain analytical expression: ,in, a i , b i , c i For the recursive convolution coefficients of the line-mode transfer function, The sampling interval is denoted as .

7. The method for single-terminal quantity protection without setting in a multi-terminal flexible DC power grid according to claim 1, characterized in that, The one or more processors are capable of processing the b M0set Set to 300kV, and the k set Set it to 0.

3.

8. The method for single-terminal quantity protection without setting in a multi-terminal flexible DC power grid according to claim 1, characterized in that, Intra-regional similarity S internal Similarity with areas outside the region S external It can be determined using the following formula: In the formula: N The number of sampling points; X i For the first i One sampling point; Y j For the first j One sampling point.

9. The method for single-terminal quantity protection without setting in a multi-terminal flexible DC power grid according to claim 1, characterized in that, The intra-zone similarity is used to characterize the similarity between the measured first traveling wave at this end and the analytical value of the first traveling wave transmitted from the line-side fault of the current-limiting reactor at the opposite end of the line to the protection point at this end. The extra-zone similarity is used to characterize the similarity between the measured first traveling wave at this end and the analytical value of the first traveling wave transmitted from the bus-side fault of the current-limiting reactor at the opposite end of the line to the protection point at this end.

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