Flexible DC Grid Protection Method Applicable to Different Boundary Structures and Computer Equipment

Abstract

The present invention discloses a flexible DC grid protection method applicable to different boundary structures, comprising the following steps: S1, collecting fault current and voltage traveling wave data, performing conversion and calculation based on the fault current and voltage traveling wave data, and judging to obtain the fault direction; S2, if the fault direction is the positive direction, judging the fault polarity to obtain the faulty pole and the healthy pole; S3, respectively calculating the main frequency component of the first traveling wave of the fault and the integral value of the voltage traveling wave of the healthy pole according to the faulty pole and the healthy pole; S4, obtaining the fault occurrence area according to the relationship between the main frequency component of the first traveling wave of the fault and the integral value of the voltage traveling wave of the healthy pole and a known threshold; S5, making a preset adjustment to the grid protection system according to the fault occurrence area. The present invention does not require artificial setting of protection thresholds, and the protection scheme is applicable to both "grids with boundaries" and "grids without boundaries" at the same time, which has important practical significance for maintaining the safe and stable operation of the flexible DC transmission grid.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible DC power grid fault protection, and particularly to a flexible DC power grid protection method applicable to different boundary structures and a computer device. Background Art

[0002] As an important part of the low-carbon operation of the power system, DC transmission technology has significant advantages such as long-distance power transmission, high transmission capacity, and flexible power control, which can effectively solve the problem of unbalanced spatial distribution of energy in China. Compared with AC power grids, flexible DC power grids are composed of numerous power electronic devices, and have characteristics such as low inertia, more complex electromagnetic transient responses, and weak short-circuit current withstand capabilities, which pose great challenges to their protection performance.

[0003] With the continuous development of power electronic technology and flexible DC transmission technology, the topological structure of flexible DC power grids will become more complex. There will be multiple connection lines between different converter stations. The "bounded power grid" with current-limiting reactors installed at both ends of the line and obvious boundaries between lines will inevitably transform into an "unbounded power grid" with current-limiting reactors installed at the outlet of the converter station and no obvious boundaries between lines. As one of the main protections for flexible DC transmission lines, studying its protection performance has very important practical significance. However, existing traveling wave protection schemes mainly rely on the change mechanism of fault characteristic quantities when passing through current-limiting reactors to achieve, and cannot guarantee the protection performance after the protection scheme is transplanted to the "unbounded power grid". Moreover, the protection thresholds of most schemes rely on a large number of simulations and lack theoretical support, which is difficult in actual engineering applications.

[0004] Therefore, there is an urgent need to study a new technical solution to solve the technical problem that the existing flexible DC power grid protection scheme is not applicable to a new type of flexible DC power grid with current-limiting reactors installed at the outlet of the converter station and no obvious boundaries between lines. Summary of the Invention

[0005] The present invention provides a flexible DC power grid protection method applicable to different boundary structures and a computer device to solve the technical problem that the existing flexible DC power grid protection scheme is not applicable to a new type of flexible DC power grid with current-limiting reactors installed at the outlet of the converter station and no obvious boundaries between lines.

[0006] To achieve the above object, the present invention provides a flexible DC power grid protection method applicable to different boundary structures, including the following steps:

[0007] S1. Collect fault current and voltage traveling wave data, perform conversion and calculation based on the fault current and voltage traveling wave data, and judge to obtain the fault direction;

[0008] S2. If the fault direction is the positive direction, judge the fault polarity to obtain the faulty pole and the healthy pole;

[0009] S3. Calculate the main frequency component of the first traveling wave of the fault and the integral value of the traveling wave of the sound pole voltage respectively according to the fault pole and the sound pole;

[0010] S4. Obtain the fault occurrence area according to the relationship between the main frequency component of the first traveling wave of the fault and the integral value of the traveling wave of the sound pole voltage and a known threshold;

[0011] S5. Make a preset adjustment to the power grid protection system according to the fault occurrence area.

[0012] Preferably, S4 includes:

[0013] Assume ω domi represents the main frequency component of the first traveling wave of the fault, λ represents the integral value of the traveling wave of the sound pole voltage, ω set represents the value of the main frequency component of the first traveling wave of the fault at the sampling point of the end of the internal area fault, and λ set represents the integral value of the traveling wave of the sound pole voltage at the sampling point of the end of the internal area fault, then:

[0014] When ω domi > ω set ∩λ < λ set is satisfied, it is determined that the fault occurrence area is within the area.

[0015] Preferably, S5 includes:

[0016] When the protection device identifies an internal area fault, it issues a DC circuit breaker trip signal and performs corresponding actions according to the fault pole:

[0017] If the fault is a positive pole grounding fault, the positive pole circuit breaker of the transmission line trips, the positive pole line is cut off, and the remaining lines operate normally; if the fault is a negative pole grounding fault, the negative pole circuit breaker of the transmission line trips, the negative pole line is cut off, and the remaining lines operate normally.

[0018] Preferably, S5 further includes:

[0019] When the protection device identifies an external area fault, the protection device of this line sends a blocking signal, and the circuit breakers at both ends of the line do not operate.

[0020] Preferably, S1 includes:

[0021] After collecting the fault current and voltage traveling wave data, perform phase-mode transformation to obtain the pole-mode and zero-mode traveling wave data. The phase-mode transformation can be expressed by the following relational formula:

[0022]

[0023] where u1, u0, i1, and i0 respectively represent the line-mode voltage, zero-mode voltage, line-mode current, and zero-mode current; u p 、un 、i p 、i n respectively represent the positive - pole voltage, negative - pole voltage, positive - pole current, and negative - pole current; S represents the phase - mode transformation matrix; among them, the first backward traveling wave after the fault is the first traveling wave of the fault;

[0024] Calculate the energy of the first forward traveling wave of the fault voltage and the energy of the first traveling wave of the fault voltage respectively. When the energy of the first traveling wave of the fault voltage is greater than the energy of the first forward traveling wave of the fault voltage, it is determined as a forward - direction fault; when the energy of the first traveling wave of the fault voltage is less than the energy of the first forward traveling wave of the fault voltage, it is determined as a reverse - direction fault:

[0025]

[0026] where E for and E ini respectively represent the energy of the first forward traveling wave of the fault voltage and the energy of the first traveling wave of the fault voltage; U for and U ini respectively represent the first forward traveling wave of the voltage and the first traveling wave of the voltage; t0 and t1 represent the sampling time points of the data window; ω represents the traveling - wave frequency of the fault voltage.

[0027] Preferably, S1 further includes:

[0028] If the fault direction is reverse, the line protection device sends a blocking signal, and the circuit breakers at both ends of the line do not operate.

[0029] Preferably, S2 includes:

[0030] Calculate the energy of the first traveling wave of the positive - pole voltage and the energy of the first traveling wave of the negative - pole voltage respectively. When the energy of the first traveling wave of the positive - pole voltage is greater than the energy of the first traveling wave of the negative - pole voltage, it is determined as a positive - pole fault; when the energy of the first traveling wave of the positive - pole voltage is less than the energy of the first traveling wave of the negative - pole voltage, it is determined as a negative - pole fault:

[0031]

[0032] where, E pos and E neg respectively represent the energy of the first traveling wave of the positive - pole voltage and the energy of the first traveling wave of the negative - pole voltage;

[0033] According to the fault polarity discrimination, the fault pole and the sound pole can be obtained.

[0034] Preferably, S3 includes:

[0035] The energy of each frequency band of the first traveling wave of the fault inside and outside the zone can be expressed by the following relational formula:

[0036]

[0037] where, e -α(ω)xThe real part of the transfer function when a fault occurs within the zone, \(x\) represents the fault distance; \(e\) -α(ω)l The real part of the transfer function when a fault occurs outside the zone, \(l\) represents the full length of the protected line; \(U_0\) represents the amplitude of the first traveling wave of the zero-mode voltage at the fault point, \(H(j\omega)\) represents the transfer function of the boundary element of the transmission line; \(E\) in Represents the energy of the first traveling wave for an internal fault; \(E\) out Represents the energy of the first traveling wave for an external fault; \(U\) f10 Represents the first traveling wave of the zero-mode voltage for an internal fault; \(U\) f20 Represents the first traveling wave of the zero-mode voltage for an external fault;

[0038] By taking the derivative of the first traveling wave energy operator expressions for internal and external faults with respect to frequency respectively, we can obtain:

[0039]

[0040] Among them, \(k\) a Represents the amplitude attenuation coefficient per unit length, \(\tau\) a Represents the distortion coefficient per unit length;

[0041] It can be seen from this that for an internal fault, when \(\omega = 1 / (\tau\) a \(x)\), there is a unique maximum value in the first traveling wave energy expression, that is, there is a maximum value for the energy of each frequency component of the first traveling wave. The frequency component corresponding to this maximum value of the first traveling wave energy is the main frequency component \(\omega\) domi ;

[0042] According to phase-mode transformation, the healthy pole voltage traveling wave is composed of a line-mode component and a zero-mode component, and its complex frequency domain expression is:

[0043]

[0044] Among them, \(s\) represents the complex variable in the complex frequency domain;

[0045] The integral value \(\lambda\) of the healthy pole voltage traveling wave can be expressed by the following relationship:

[0046]

[0047] Among them, \(t\) represents any moment within the range of the healthy pole voltage traveling wave; \(T\) represents the length of the sliding time window; \(U\) n Represents the healthy pole voltage traveling wave.

[0048] The present invention also provides a computer device, including a processor, a memory, and a computer program for implementing the method of the present invention.

[0049] The present invention has the following beneficial effects:

[0050] The flexible DC grid protection method applicable to different boundary structures of the present invention deeply studies and analyzes the voltage and current traveling waves after a fault. The protection scheme of this method does not rely on the change mechanism of fault characteristic quantities when passing through a current-limiting reactor to achieve, but proposes two new criteria, namely the main frequency component of the first traveling wave of the fault and the integral value of the healthy pole voltage traveling wave, and combines the two. According to the relationship between the two and known thresholds, the fault occurrence area is judged, so that this method does not require artificial setting of protection thresholds and the protection scheme is applicable to both "grid with boundaries" and "grid without boundaries" at the same time, which has important practical significance for maintaining the safe and stable operation of the flexible DC transmission grid.

[0051] In addition to the purposes, features and advantages described above, the present invention has other purposes, features and advantages. The following will refer to the accompanying drawings and make a further detailed description of the present invention. Brief Description of the Drawings

[0052] The accompanying drawings forming a part of this application are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0053] Figure 1 is a schematic flow chart of the first preferred embodiment of the present invention.

[0054] Figure 2 is a schematic diagram of the topology structure of a flexible DC transmission grid with boundaries in the second preferred embodiment of the present invention.

[0055] Figure 3 is a schematic diagram of the topology structure of a flexible DC transmission grid without boundaries in the second preferred embodiment of the present invention.

[0056] Figure 4 is a schematic cross-sectional view of the flexible DC transmission line structure in the second preferred embodiment of the present invention. Detailed Description of the Embodiments

[0057] The following will make a detailed description of the embodiments of the present invention with reference to the accompanying drawings. However, the present invention can be implemented in many different ways defined and covered by the claims.

[0058] Embodiment 1:

[0059] In the preferred embodiment of the present invention, a flexible DC grid protection method applicable to different boundary structures is provided, including the following steps:

[0060] S1. Collect the fault current and voltage traveling wave data, perform conversion and calculation based on the fault current and voltage traveling wave data, and judge the fault direction; S1 specifically includes:

[0061] After collecting the fault current and voltage traveling wave data, phase-mode transformation is performed to obtain the line-mode and zero-mode traveling wave data. The phase-mode transformation can be expressed by the following relational expressions:

[0062]

[0063] where u1, u0, i1, and i0 represent the line-mode voltage, zero-mode voltage, line-mode current, and zero-mode current respectively; u p , u n , i p , and i n represent the positive-pole voltage, negative-pole voltage, positive-pole current, and negative-pole current respectively; S represents the phase-mode transformation matrix; where the first backward traveling wave after the fault is the fault initial traveling wave;

[0064] Calculate the energy of the first forward traveling wave of the fault voltage and the energy of the fault voltage initial traveling wave respectively. When the energy of the fault voltage initial traveling wave is greater than the energy of the first forward traveling wave of the fault voltage, it is determined as a forward-direction fault; when the energy of the fault voltage initial traveling wave is less than the energy of the first forward traveling wave of the fault voltage, it is determined as a reverse-direction fault:

[0065]

[0066]

[0067] where E for and E ini represent the energy of the first forward traveling wave of the fault voltage and the energy of the fault voltage initial traveling wave respectively; U for and U ini represent the first forward traveling wave of the voltage and the voltage initial traveling wave respectively; t0, t1 represent the data window sampling time points; ω represents the fault voltage traveling wave frequency.

[0068] S1 also includes: If the fault direction is reverse, the protection device of this line sends a blocking signal, and the circuit breakers at both ends of the line do not operate.

[0069] S2. If the fault direction is forward, judge the fault polarity to obtain the fault pole and the healthy pole; S2 specifically includes:

[0070] Calculate the energy of the initial traveling wave of the positive-pole voltage and the energy of the initial traveling wave of the negative-pole voltage respectively. When the energy of the initial traveling wave of the positive-pole voltage is greater than the energy of the initial traveling wave of the negative-pole voltage, it is determined as a positive-pole fault; when the energy of the initial traveling wave of the positive-pole voltage is less than the energy of the initial traveling wave of the negative-pole voltage, it is determined as a negative-pole fault:

[0071]

[0072] where, E pos and E neg represent the energy of the initial traveling wave of the positive-pole voltage and the energy of the initial traveling wave of the negative-pole voltage respectively;

[0073] By distinguishing according to the fault polarity, the faulty pole and the sound pole can be obtained. In the preferred embodiment of the present invention, judging the faulty pole and the sound pole can clarify the faulty pole for the subsequent calculation process.

[0074] S3. Calculate the main frequency component of the first traveling wave of the fault and the integral value of the traveling wave voltage of the sound pole respectively according to the faulty pole and the sound pole; S3 specifically includes:

[0075] The energies of the first traveling waves in different frequency bands for internal and external faults can be expressed by the following relational expressions:

[0076]

[0077] Among them, e -α(ω)x represents the real part of the transfer function during internal fault, x represents the fault distance; e -α(ω)l represents the real part of the transfer function during external fault, l represents the total length of the protected line; U0 represents the amplitude of the first traveling wave of the zero-mode voltage at the fault point, H(jω) represents the transfer function of the transmission line boundary element; E in represents the energy of the first traveling wave during internal fault; E out represents the energy of the first traveling wave during external fault; U f10 represents the first traveling wave of the zero-mode voltage during internal fault; U f20 represents the first traveling wave of the zero-mode voltage during external fault;

[0078] Taking the derivative of the first traveling wave energy operator expressions for internal and external faults with respect to frequency respectively, we can get:

[0079]

[0080] Among them, k a represents the amplitude attenuation coefficient per unit length, τ a represents the distortion coefficient per unit length;

[0081] It can be seen from this that for internal faults, when ω = 1 / (τ a x), there is a unique maximum value in the first traveling wave energy expression, that is, there is a maximum value in the energies of the first traveling wave frequency components. The frequency component corresponding to this maximum value of the first traveling wave energy is the main frequency component ω domi of the first traveling wave of the fault; as the fault distance increases, the maximum value point of the first traveling wave energy of the fault gradually approaches the low frequency, that is, the main frequency component of the first traveling wave of the fault gradually monotonically decays to the low frequency. When an external fault occurs and passes through the boundary element, the main frequency component of the first traveling wave of the fault will show a significant attenuation.

[0082] According to the phase-mode transformation, the traveling wave voltage of the sound pole is composed of the line-mode component and the zero-mode component, and its complex frequency domain expression is:

[0083]

[0084] where s represents a complex variable in the complex frequency domain;

[0085] Due to the difference in wave velocities between the line-mode traveling wave and the zero-mode traveling wave, generally the wave velocity of the line-mode traveling wave is faster than that of the zero-mode traveling wave. Therefore, when the fault distance changes, the time difference between the arrival times of the two traveling wave components also changes. Moreover, as the fault distance changes, the amplitude of the voltage traveling wave is also affected by the attenuation coefficient of the transmission line. Based on this series of differences, internal and external faults can be effectively identified. By integrating the healthy-pole voltage traveling wave within a sliding time window to obtain an integral value, the difference between internal and external faults can be effectively amplified, realizing the effective identification of internal and external faults. The integral value λ of the healthy-pole voltage traveling wave can be expressed by the following relational expression:

[0086]

[0087] where t represents any moment within the range of the healthy-pole voltage traveling wave; T represents the length of the sliding time window, generally with a value not less than 1 ms; U n represents the healthy-pole voltage traveling wave.

[0088] To avoid the influence of noise on the integral value of the healthy-pole voltage traveling wave, the measured healthy-pole voltage traveling wave data is first normalized and then the integral value is calculated. Different from the main frequency component ω domi of the first fault traveling wave, the integral value λ of the healthy-pole voltage traveling wave increases with the increase of the fault distance. When an external fault occurs and passes through the boundary element, the integral value of the healthy-pole voltage traveling wave still remains increasing. By combining the two fault criteria with opposite fault characteristics, namely the main frequency component ω domi of the first fault traveling wave and the integral value λ of the healthy-pole voltage traveling wave, the complementary advantages of intervals can be realized, thereby expanding the protection applicability and protection range.

[0089] S4. Obtain the fault occurrence area according to the relationship between the main frequency component of the first fault traveling wave and the integral value of the healthy-pole voltage traveling wave and the known threshold; S4 specifically includes:

[0090] Assume that ω domi represents the main frequency component of the first fault traveling wave, λ represents the integral value of the healthy-pole voltage traveling wave, ω set represents the value of the main frequency component of the first fault traveling wave at the sampling point of the internal terminal fault, and λ set represents the integral value of the healthy-pole voltage traveling wave at the sampling point of the internal terminal fault. Then:

[0091] When ω domi > ω set ∩λ < λ set is satisfied, it is determined that the fault occurrence area is within the zone; otherwise, it is determined that the fault occurrence area is outside the zone.

[0092] In the preferred embodiment of the present invention, the adopted ωset and λ set The criterion can be determined only through one simulation, without the need for a large number of simulation experiments as in traditional methods, and without the need for artificial threshold setting.

[0093] S5. Make preset adjustments to the power grid protection system according to the fault occurrence area. S5 specifically includes:

[0094] When the protection device identifies an in-zone fault, it issues a DC circuit breaker tripping signal and performs corresponding actions according to the fault pole:

[0095] If the fault is a positive pole grounding fault, the positive pole circuit breaker of the transmission line trips, the positive pole line is cut off, and the other lines operate normally; if the fault is a negative pole grounding fault, the negative pole circuit breaker of the transmission line trips, the negative pole line is cut off, and the other lines operate normally.

[0096] When the protection device identifies an out-of-zone fault, the protection device of this line sends a blocking signal, and the circuit breakers at both ends of the line do not operate.

[0097] To further illustrate the beneficial effects of the method of the present invention, the protection schemes of existing references are selected for comparative analysis. The simulation test results are shown in Table 1. In the protection schemes of existing references, the polarity characteristics of the voltage traveling waves at both ends of the line during in-zone and out-of-zone faults are utilized, and on this basis, a traveling wave protection scheme based on voltage polarity comparison is proposed, which can be applied to flexible DC power grids with two different boundary structures. Compared with the protection schemes of existing references, the scheme of the present invention uses both the fault pole and the healthy pole information, and combines two fault criteria with opposite fault characteristics for fault discrimination, greatly improving the ability of the protection scheme to withstand transition resistance and enhancing the protection performance. In addition, the protection schemes of existing references use double-ended quantities for protection, and the traveling wave data of the protection devices at both ends of the transmission line need to be used simultaneously, which will be affected by the communication at both ends and increase the protection cost. However, the scheme of the present invention uses single-ended quantity for protection, without considering communication problems and with a protection cost lower than that of the existing reference protection scheme.

[0098] Table 1 Comparative test simulation results of the scheme of the present invention and the existing reference scheme

[0099]

[0100]

[0101] As can be seen from the above table results, the protection schemes of existing references have poor applicability in "borderless power grids" and limited ability to withstand transition resistance when applied to "power grids with boundaries", proving that the protection performance of the scheme of the present invention is more excellent.

[0102] A preferred embodiment of the present invention further provides a computer device, including a processor, a memory, and a computer program for implementing the method of the present invention.

[0103] Embodiment 2:

[0104] In a preferred embodiment of the present invention, taking the Zhangbei flexible DC grid as an actual engineering reference, a ±500 kV flexible DC transmission grid model with and without boundaries as shown in Figure 2 and Figure 3 is built on the PSCAD / EMTDC software. In Figure 2 and Figure 3 , MMC1, MMC2, MMC3, and MMC4 respectively represent Converter Station No. 1, Converter Station No. 2, Converter Station No. 3, and Converter Station No. 4, and G1, G2, G3, and G4 respectively represent AC Source No. 1, AC Source No. 2, AC Source No. 3, and AC Source No. 4; the overhead lines adopt a frequency-variable parameter model, and the lengths of each line are 205.9 km, 188.1 km, 208.4 km, and 49.6 km respectively. For the structural profile and spatial parameters of the flexible DC transmission line, refer to Figure 4 and Table 2; in Figure 4 , C1 and C2 respectively represent Conductor 1 and Conductor 2, W1 and W2 respectively represent Ground Wire 1 and Ground Wire 2, W represents the ground, the height of the lowest conductor relative to the ground is 30 m, the horizontal distance between the conductors is 10 m, the height of the ground wire is more than 10 m from the lowest conductor, the horizontal distance between the ground wires is 10 m, and the tower type is DC2. The basic information of each converter station is shown in Table 3, and the value of the current-limiting reactor is uniformly set to 200 mH. Taking the failure of the 205.9 km transmission line as an example, the simulation results of the protection ability test of the method of the present invention are shown in Table 4.

[0105] Table 2 Spatial Parameters of Flexible DC Transmission Line

[0106]

[0107] Table 3 Basic Information of Each Converter Station

[0108]

[0109] Table 4 Simulation Results of Protection Ability Test of the Method of the Present Invention

[0110]

[0111]

[0112] Analyzing the protection results, it can be seen that the method of the present invention can still operate reliably under the condition of a 500 Ω transition resistance and is applicable to flexible DC grids with different boundary structures.

[0113] In summary, the flexible DC grid protection method applicable to different boundary structures in the preferred embodiment of the present invention deeply studies and analyzes the voltage and current traveling waves after a fault. This method does not rely on the change mechanism of fault characteristic quantities when passing through a current-limiting reactor to achieve. Two new criteria, namely the main frequency component of the first traveling wave of the fault and the integral value of the healthy pole voltage traveling wave, are proposed and combined. According to the relationship between the two and the known thresholds, the fault occurrence area is judged, so that this method does not require artificial setting of protection thresholds and the protection scheme is applicable to both "grids with boundaries" and "grids without boundaries" at the same time, which has important practical significance for maintaining the safe and stable operation of the flexible DC transmission grid.

[0114] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A flexible DC power grid protection method applicable to different boundary structures, characterized in that: The following steps are involved: S1. Collecting fault current and voltage traveling wave data, converting and calculating based on the fault current and voltage traveling wave data, and determining the fault direction; including: After collecting the fault current and voltage traveling wave data, phase mode transformation is performed to obtain the polar mode and zero mode traveling wave data. The phase mode transformation can be expressed by the following relationship: Where u1, u0, i1, i0 represent line mode voltage, zero mode voltage, line mode current, zero mode current respectively; u p 、u n 、i p 、i n They represent positive voltage, negative voltage, positive current, and negative current respectively; S represents the phase mode transformation matrix; the first reverse wave after the fault is the first fault wave; The energy of the first fault voltage wave before and the first fault voltage wave before is calculated respectively. When the energy of the first fault voltage wave before is greater than the energy of the first fault voltage wave before, it is determined to be a positive direction fault. When the energy of the first fault voltage wave before is less than the energy of the first fault voltage wave before, it is determined to be a reverse direction fault: Where E for and E ini They represent the energy of the first fault voltage traveling wave and the energy of the first fault voltage traveling wave respectively; U for and U ini They represent the first voltage front wave and the first voltage wave respectively; t0 and t1 represent the data window sampling time points; ω represents the fault voltage wave frequency; S2. If the fault direction is the positive direction, determine the fault polarity to obtain the fault pole and the sound pole; including: Calculate the first wave energy of the positive and negative voltages respectively. When the first wave energy of the positive voltage is greater than that of the negative voltage, it is determined to be a positive fault. When the first wave energy of the positive voltage is less than that of the negative voltage, it is determined to be a negative fault: Among them, E pos and E neg Respectively represent the first wave energy of the positive voltage and the first wave energy of the negative voltage; According to the fault polarity, the fault pole and the sound pole are obtained; S3, respectively calculating the main frequency component of the first traveling wave of the fault and the integral value of the traveling wave of the voltage of the sound pole according to the fault pole and the sound pole; S4. Obtaining the fault occurrence area according to the relationship between the main frequency component of the first traveling wave of the fault and the integral value of the healthy pole voltage traveling wave and a known threshold, including: Assume ω domi represents the main frequency component of the first traveling wave of the fault, λ represents the integral value of the traveling wave of the healthy pole voltage, ω set represents the main frequency component value of the first fault wave at the end fault sampling point in the zone, λ set represents the traveling wave integral value of the sound pole voltage at the terminal fault sampling point in the zone, then: When ω is satisfied domi >ω set ∩λ<λ set When , it is determined that the fault occurs in the area within the zone; S5. Make preset adjustments to the power grid protection system according to the fault occurrence area.

2. The flexible DC power grid protection method applicable to different boundary structures according to claim 1 is characterized in that: The S5 includes: When the protection device identifies a fault in the zone, it sends a DC circuit breaker trip signal and takes corresponding actions according to the fault pole: If the fault is a positive pole grounding fault, the positive pole circuit breaker of the transmission line will trip, the positive pole line will be cut off, and the remaining lines will operate normally; if the fault is a negative pole grounding fault, the negative pole circuit breaker of the transmission line will trip, the negative pole line will be cut off, and the remaining lines will operate normally.

3. The flexible DC power grid protection method applicable to different boundary structures according to claim 2 is characterized in that: The S5 further includes: When the protection device identifies an out-of-zone fault, the line protection device sends a blocking signal and the circuit breakers at both ends of the line do not operate.

4. The flexible DC power grid protection method applicable to different boundary structures according to claim 3 is characterized in that: The S1 further comprises: If the fault direction is the reverse direction, the line protection device sends a blocking signal and the circuit breakers at both ends of the line do not operate.

5. The flexible DC power grid protection method applicable to different boundary structures according to claim 4 is characterized in that: The S3 includes: The energy of each frequency band of the first wave of internal and external faults can be expressed by the following relationship: Among them, e -α(ω)x represents the real part of the transmission function when the fault occurs within the area, x represents the fault distance; e -α(ω)l represents the real part of the transmission function when there is an out-of-zone fault, l represents the total length of the protection line; U0 represents the amplitude of the first wave of the zero-mode voltage at the fault point, H(jω) represents the transfer function of the boundary element of the transmission line; E in Indicates the energy of the first traveling wave of the fault in the area; E out Indicates the energy of the first traveling wave of the out-of-zone fault; U f10 Indicates the first wave of zero-mode voltage in the fault zone; U f20 It represents the first wave of zero-mode voltage of out-of-zone fault; By using the first wave energy operator expressions of the internal and external faults to differentiate the frequency, we can obtain: Among them, k a Represents the amplitude attenuation coefficient per unit length, τ a Represents the distortion coefficient per unit length; From this we can know that: when the fault is within the area, ω=1 / (τ a x), the energy expression of the first fault wave has a unique maximum value, that is, the energy of each frequency component of the first fault wave has a maximum value, and the frequency component corresponding to the maximum energy of the first fault wave is the main frequency component ω of the first fault wave. domi ; According to the phase mode transformation, the healthy pole voltage traveling wave is composed of line mode component and zero mode component, and its complex frequency domain expression is: Where s represents a complex variable in the complex frequency domain; The traveling wave integral value λ of the healthy pole voltage is expressed by the following relationship: Where t represents any time within the range of the healthy pole voltage traveling wave; T represents the length of the sliding time window; U n Represents a healthy pole voltage traveling wave.

6. A computer device, characterized in that: The invention comprises a processor, a memory and a computer program for implementing the method according to any one of claims 1 to 5.

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