Complex line low-current single-phase earth fault line selection method based on harmonic amplitude ratio break variable

Abstract

The invention discloses a complex line low-current single-phase earth fault line selection method based on a harmonic amplitude ratio break variable. The method comprises the following steps: S1, synchronously collecting three-phase current signals of each line of a power distribution network; s2, carrying out harmonic analysis, and extracting amplitude data; s3, calculating the harmonic amplitude ratio Kn of the split-phase fundamental current corresponding to the amplitude data of the harmonic current; s4, transversely comparing the harmonic amplitude ratio Kn, and calculating the break variable delta Kn of the harmonic amplitude ratio; s5, determining a line selection criterion threshold value delta Kn0; s6, judging whether the zero harmonic amplitude ratio break variable delta K0 is abnormally changed or not, and if not, returning to the step S1; if the mark is delta K0, entering the step S7; s7, comparing the delta K0 variation with the zero-order harmonic amplitude ratio break variable delta K0 of all the lines in the longitudinal direction, and if the delta K0 variation is not the maximum value, returning to the step S1; and if the delta K0 becomes the maximum value, the fault phase of the fault line is determined. According to the line selection method, the single-phase earth fault can be accurately diagnosed, and accurate line selection can be achieved even in a system with a complex line.

Description

Technical Field

[0001] The invention belongs to the technical field of power system and automation thereof, in particular to a method for selecting a complex line small current single-phase ground fault based on a sudden change amount of harmonic amplitude ratio. Background Art

[0002] In my country, distribution networks generally use low-current grounding to improve power supply stability. However, among the various faults that occur in distribution networks, single-phase grounding faults are the most common, accounting for up to 80%. This fault not only causes system voltage reduction and system imbalance, but also may cause equipment damage and, in severe cases, electrical fires, posing a potential threat to personal safety. It is worth noting that the characteristics of single-phase grounding faults are usually relatively weak, making it crucial to quickly make accurate judgments and select the faulty line after the fault occurs. This can quickly restore power supply, reduce power outage time, and help protect equipment and personnel safety and maintain the stable operation of the power system. Therefore, it is of great significance to study fault diagnosis methods for single-phase grounding faults. These methods can not only improve the reliability and stability of the distribution network, but also reduce maintenance costs and improve the power supply quality to a certain extent. By diagnosing single-phase grounding faults in a timely and accurate manner, distribution network operators can quickly and accurately take corresponding measures to minimize the impact of faults on the power supply system and ensure the continuity and stability of power supply.

[0003] In the field of fault diagnosis and line selection of power systems, steady-state methods and transient methods have always been the main research directions. Most methods can improve the accuracy of fault line selection, but they only analyze systems containing a single line. Cable-overhead line hybrid lines are also used in small current grounding systems due to their flexibility and durability. Due to the differences in the distribution parameters of cable lines and overhead lines, traditional fault line selection methods may fail at this time. Therefore, for such complex systems, it is necessary to explore more advanced fault diagnosis and line selection methods. In this context, real-time monitoring of power quality data has become an indispensable task. By monitoring and analyzing key data such as voltage, current parameters and harmonics, abnormal conditions in the system can be quickly captured, and possible fault sources can be quickly located. This targeted diagnostic method can take repair measures more quickly, thereby minimizing power outage time and production losses during system operation. In addition, combined with the relevant data of each harmonic for comprehensive analysis, the fault type and location can be more accurately determined, further improving the accuracy and reliability of diagnosis. This method of comprehensive data utilization helps to reduce the impact of other interference factors on line selection accuracy. Therefore, the fault diagnosis and line selection method based on power quality data provides new ideas and solutions for solving fault problems in complex systems. Summary of the invention

[0004] The purpose of the present invention is to provide a complex line small current single-phase grounding fault selection method based on the harmonic amplitude ratio mutation amount in order to solve the problems existing in the prior art.

[0005] The purpose of the present invention is to be solved by the following technical solutions:

[0006] A method for selecting a complex line with a small current single-phase ground fault based on a sudden change in harmonic amplitude ratio, characterized in that the method has the following steps:

[0007] S1. Set the sampling frequency to synchronously collect the three-phase current signals of each line of the distribution network;

[0008] S2, setting a harmonic analysis period, performing harmonic analysis on each phase of the three-phase current signal collected in step S1, and extracting the amplitude data of the harmonic current of each phase within each harmonic analysis period;

[0009] S3, based on the amplitude data of each phase harmonic current obtained in step S2, calculate the harmonic amplitude ratio K of the phase fundamental current corresponding to the amplitude data of different phase harmonic currents in each harmonic analysis cycle of each line. n ;

[0010] S4, harmonic amplitude ratio K obtained in step S3 n , horizontally compare the harmonic amplitude ratio K of two adjacent harmonic analysis cycles on the same phase of the same line n , calculate the harmonic amplitude ratio mutation ΔK n ;

[0011] S5. Determine the harmonic amplitude ratio mutation value ΔK based on the difference in harmonic characteristics between the faulty line and the non-faulty line. n The line selection criterion threshold ΔK n0 ;

[0012] S6: Compare the line selection criterion threshold ΔK obtained in step S5 n0 The 0th harmonic amplitude ratio mutation amount ΔK0 of each line obtained in step S4 is used to determine whether the 0th harmonic amplitude ratio mutation amount ΔK0 has an abnormal change. If not, return to step S1; if an abnormal change occurs, it is determined that a small current grounding fault has occurred and marked as ΔK 0变 , proceed to step S7;

[0013] S7. Change the 0th harmonic amplitude of the abnormal phase of the line with abnormal change to the sudden change value ΔK 0变 The vertical harmonic amplitude of the entire line is compared with the sudden change ΔK0. If ΔK 0变 Not the maximum value, return to step S1; if ΔK 0变 is the maximum value, then the abnormal phase of the line with the abnormal change is determined to be the fault phase of the fault line.

[0014] The three-phase current signal in step S1 is obtained through a current transformer at the voltage bus outlet.

[0015] The sampling frequency in step S1 is set to 12.8 kHz.

[0016] The harmonic analysis period in step S2 is 0.02s.

[0017] The harmonic analysis in step S2 adopts the Fourier series transform method:

[0018]

[0019] Among them, i(t) is the current signal of one phase of the three-phase current of a certain line, ω is the angular velocity of the current of this phase under the power frequency of 50Hz, I0 is the amplitude of the zero-order harmonic current of this phase, also known as the DC component, I n It represents the amplitude of the nth harmonic current of this phase, t is the actual time, is the phase angle of the nth harmonic current of this phase, and n is the harmonic order.

[0020] The harmonic amplitude ratio K in step S3 n The calculation method is:

[0021]

[0022] Among them, I1 is the fundamental current amplitude of a phase of a line in a certain harmonic analysis period, that is, the 1st harmonic current amplitude in the corresponding harmonic analysis period. The harmonic amplitude ratio can be obtained by the ratio of the nth amplitude of the harmonic current of each phase of different lines to the amplitude of the fundamental current.

[0023] The harmonic amplitude ratio mutation amount ΔK in step S4 n The calculation method is:

[0024] ΔK n =K n(k+1) -K n(k) (3)

[0025] Where K n(k) and K n(k+1) They respectively represent the harmonic amplitude ratios of the phase-by-phase fundamental current corresponding to the amplitude data of the nth harmonic current of a certain phase of a certain line in the previous and next adjacent harmonic analysis periods.

[0026] The line selection criterion threshold ΔK in step S5 n0 The determination method is as follows: for any phase of any line within a certain harmonic analysis period, the amplitude of the 0th harmonic is 5% of the sudden change ΔK0 as the line selection criterion threshold ΔK n0 , preferably 3%.

[0027] The longitudinal direction in step S7 refers to the same harmonic analysis period.

[0028] The device used in the line selection method includes:

[0029] Current acquisition module, used to collect three-phase current signals of each line in the system;

[0030] Harmonic analysis module, used to perform harmonic analysis on the three-phase current signals of each line;

[0031] A line selection judgment module is used to judge the fault line according to the change of the harmonic amplitude ratio mutation amount;

[0032] The result output module is used to output the judgment result of the fault line.

[0033] The device used in the line selection method also includes a 5G communication module, which is used to transmit the judgment results of the faulty line to a remote monitoring center.

[0034] Compared with the prior art, the present invention has the following advantages:

[0035] The line selection method of the present invention does not require the use of other complex equipment, and comprehensively utilizes the relevant data of each harmonic for analysis, which helps to reduce the influence of other interference factors on line selection, improves the accuracy and reliability of diagnosing single-phase grounding faults, and can accurately select lines even in systems with complex lines. It has excellent practicality and provides an efficient and economical solution for fault diagnosis and line selection of power systems. It also brings a more convenient and sustainable management method to the development of the power industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Attached Figure 1 A flow chart of a complex line small current single-phase grounding fault line selection method based on the harmonic amplitude ratio mutation amount provided by the present invention;

[0037] Attached Figure 2 A basic principle diagram of a complex line small current single-phase grounding fault line selection method based on the harmonic amplitude ratio mutation provided by the present invention;

[0038] Attached Figure 3 A system principle block diagram of the fault line selection of the simulation example provided by the present invention;

[0039] Attached Figure 4 This is a system simulation diagram of the fault line selection of the simulation example provided by the present invention. DETAILED DESCRIPTION

[0040] The technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments.

[0041] The technical principle provided by the present invention is as follows: A complex line small current grounding single-phase grounding fault selection method based on the harmonic amplitude ratio mutation amount mainly utilizes the harmonic amplitude ratio mutation amount ΔK of the fault phase after the fault occurs. n Fault line selection is performed when obvious abnormal changes occur. When a single-phase grounding fault occurs, a low-impedance path is formed, the system symmetry is destroyed, and the current can flow quickly to the ground through the fault point; the grounding fault changes the current path, and it does not pass through the original complete loop but flows to the ground in advance through the fault point. At this time, the harmonic impedance of the fault path will be significantly reduced compared to before the fault, and the voltage remains unchanged. Therefore, the amplitude of each harmonic current after the fault will increase compared to before the fault, while the non-fault phase current path is almost unaffected, so the amplitude of each harmonic current does not change significantly. Since there are multiple lines with different harmonic contents in the distribution network, the judgment of the fault may be interfered with based on the amplitude of the harmonic current of the fault phase alone. Therefore, the fundamental current amplitude corresponding to each phase current of each line is selected as the reference, and the amplitudes of harmonics of different orders in one cycle are compared with the fundamental amplitude. The mutation amount of the harmonic amplitude ratio obtained in two adjacent cycles is used as the criterion for fault line selection. The basic principle of fault line selection based on the mutation amount of the harmonic amplitude ratio based on which this technical principle is based is as follows Figure 2 shown.

[0042] There are generally background harmonics when the system is running. Assume that the voltage u(t) at the common connection point when the system is operating normally is:

[0043]

[0044] Among them, U n is the amplitude of the nth harmonic voltage at the common connection point, ω u is the voltage angular velocity at 50Hz, n is the harmonic order, is the nth harmonic voltage phase angle.

[0045] Then the current of any phase of a line can be i(t):

[0046]

[0047] Among them, i(t) is the current signal of one phase of the three-phase current of a certain line, ω is the angular velocity of the current of this phase under the power frequency of 50Hz, I0 is the amplitude of the zero-order harmonic current of this phase, also known as the DC component, I n It represents the amplitude of the nth harmonic current of this phase, t is the actual time, is the phase angle of the nth harmonic current of this phase, and n is the harmonic order.

[0048] I n It can be expressed as:

[0049]

[0050] Among them, U n is the nth harmonic voltage amplitude at the common connection point, Z n is the nth harmonic impedance of the normal current path, R, L, C are the impedance parameters of the normal current path, ω z is the impedance angular velocity at an industrial frequency of 50 Hz, and n is the harmonic order.

[0051] When a single-phase grounding fault occurs in the line where i(t) is located, a low-impedance fault current path will be formed, the system symmetry will be destroyed, and the current can flow quickly to the ground through the fault point. The amplitude of the nth harmonic current of the fault phase at this time is I nf :

[0052]

[0053] Among them, Z nf is the nth harmonic impedance of the fault current path, R′, L′ and C′ are the impedance parameters of the fault current path, ω z is the impedance angular velocity at the power frequency of 50Hz, and n is the harmonic order; the ground fault changes the current path, and the current flows to the ground in advance through the fault point instead of the original complete loop. At this time, the impedance parameters R′, L′ and C′ of the fault current path are all smaller than the impedance parameters R, L and C of the normal current path, that is, Z nf Less than Z n , U n unchanged, at this time, the amplitude of each harmonic current of the fault phase after the fault will be larger than that before the fault; because each phase of each line of the distribution network contains different harmonic contents in the same harmonic analysis cycle, the judgment of the fault may be interfered with only based on the amplitude of the harmonic current of the fault phase. Therefore, the amplitude of the fundamental current of each phase corresponding to the harmonic current of each phase of each line is selected as the reference, and the ratio of the amplitude of the harmonic current of different orders to the harmonic amplitude of the fundamental current in a harmonic analysis cycle is K n :

[0054]

[0055] Where I1 is the fundamental current amplitude of a certain phase of a line in a certain harmonic analysis period, that is, the 1st harmonic current amplitude in the corresponding harmonic analysis period. The harmonic amplitude ratio K can be obtained by the ratio of the nth amplitude of each phase harmonic current of different lines to the fundamental amplitude. n .

[0056] At this time, let ΔK n Indicates the harmonic amplitude ratio K n The harmonic amplitude ratio mutation amount:

[0057] ΔK n =K n(k+1) -Kn(k)

[0058] Where K n(k) and K n(k+1) They respectively represent the harmonic amplitude ratios of the phase-by-phase fundamental current corresponding to the amplitude data of the nth harmonic current of a certain phase of a certain line in the previous and next adjacent harmonic analysis periods.

[0059] Based on the above analysis, a fault line selection method based on the harmonic amplitude ratio mutation can be established. Figure 3 Shown is the system principle block diagram of fault line selection.

[0060] The present invention provides a complex line small current single-phase grounding fault line selection method based on the harmonic amplitude ratio mutation amount, and the line selection method has the following steps:

[0061] S1, with a sampling frequency of 12.8kHz, synchronously collect the three-phase current signals of each line of the distribution network through the current transformer at the voltage bus outlet;

[0062] S2. The three-phase current signal collected in step S1 is subjected to harmonic analysis with a harmonic analysis period of 0.02s, and each phase of the three-phase current signal is subjected to harmonic analysis, and the amplitude data of the harmonic current of each phase within each harmonic analysis period are extracted; wherein the harmonic analysis adopts the Fourier series transform method:

[0063]

[0064] Among them, i(t) is the current signal of one phase of the three-phase current of a certain line, ω is the angular velocity of the current of this phase under the power frequency of 50Hz, I0 is the amplitude of the zero-order harmonic current of this phase, also known as the DC component, I n It represents the amplitude of the nth harmonic current of this phase, t is the actual time, is the phase angle of the nth harmonic current of the phase, n is the harmonic order;

[0065] S3, based on the amplitude data of each phase harmonic current obtained in step S2, calculate the harmonic amplitude ratio K of the phase fundamental current corresponding to the amplitude data of different phase harmonic currents in each harmonic analysis cycle of each line. n ; Harmonic amplitude ratio K n The calculation method is:

[0066]

[0067] Wherein, I1 is the fundamental current amplitude of a certain phase of a certain line in a certain harmonic analysis period, that is, the 1st harmonic current amplitude in the corresponding harmonic analysis period. The harmonic amplitude ratio can be obtained by the ratio of the nth amplitude of the harmonic current of each phase of different lines to the amplitude of the fundamental current;

[0068] S4, harmonic amplitude ratio K obtained in step S3 n , horizontally compare the harmonic amplitude ratio K of two adjacent harmonic analysis cycles on the same phase of the same line n , calculate the harmonic amplitude ratio mutation ΔK n ; Harmonic amplitude ratio mutation ΔK n The calculation method is:

[0069] ΔK n =K n(k+1) -K n(k) (3)

[0070] Where K n(k) and K n(k+1) They respectively represent the harmonic amplitude ratio of the phase-by-phase fundamental current corresponding to the amplitude data of the nth harmonic current of a certain phase of a certain line in the preceding and following adjacent harmonic analysis cycles;

[0071] S5. Determine the harmonic amplitude ratio mutation value ΔK based on the difference in harmonic characteristics between the faulty line and the non-faulty line. n The line selection criterion threshold ΔK n0 , line selection criterion threshold ΔK n0 The determination method is as follows: for any phase of any line within a certain harmonic analysis period, the amplitude of the 0th harmonic is 5% of the sudden change ΔK0 as the line selection criterion threshold ΔK n0 , preferably 3%;

[0072] S6: Compare the line selection criterion threshold ΔK obtained in step S5 n0 The 0th harmonic amplitude ratio mutation amount ΔK0 of each line obtained in step S4 is used to determine whether the 0th harmonic amplitude ratio mutation amount ΔK0 has an abnormal change. If not, return to step S1; if an abnormal change occurs, it is determined that a small current grounding fault has occurred and marked as ΔK 0变 , proceed to step S7;

[0073] S7. Change the 0th harmonic amplitude of the abnormal phase of the line with abnormal change to the sudden change value ΔK 0变 The vertical harmonic amplitude of the entire line is compared with the sudden change ΔK0. If ΔK 0变 Not the maximum value, return to step S1; if ΔK 0变 is the maximum value, then the abnormal phase of the line with the abnormal change is determined to be the fault phase of the fault line.

[0074] The device used in the line selection method includes: a current acquisition module, which is used to collect the three-phase current signals of each line in the system; a harmonic analysis module, which is used to perform harmonic analysis on the three-phase current signals of each line; a line selection judgment module, which is used to judge the faulty line according to the change of the harmonic amplitude ratio mutation amount; a result output module, which is used to output the judgment result of the faulty line; and also includes a 5G communication module, which is used to transmit the judgment result of the faulty line to a remote monitoring center.

[0075] Simulation Example

[0076] The following is combined with Figure 3 and attached Figure 4 The specific implementation steps of the present invention are further described.

[0077] according to Figure 3 The system principle block diagram of the fault line selection provided by the present invention is shown in the following figure. The simulation of the line selection method provided by the present invention is built in Matlab / Simulnk. Figure 4 As shown, it includes cable-overhead hybrid line L1, overhead line L2 and overhead line L3. Randomly select a phase from the three lines and set a single-phase grounding fault at any position of the phase away from the busbar. The simulation results are as follows:

[0078]

[0079] Table 1 sets the fault location on line L1 a 1 km away from the busbar

[0080] It can be seen from Table 1 that when the fault is located on the L1 line a phase, the amplitude of the 0th harmonic of the L1 line a phase has a sudden change of ΔK0 of 27.32%, which exceeds the line selection criterion threshold ΔK set to 5%. n0 , so it is judged as the fault phase. The amplitude ratio mutation of the 0th harmonic of each phase other than this does not exceed the judgment threshold, so it is judged to be normal. At this time, the line selection is correct.

[0081]

[0082] Table 2 sets the fault location on line L2 c, 6 km away from the busbar

[0083] It can be seen from Table 2 that when the fault is located on the L2 line c phase, the amplitude of the 0th harmonic of the L2 line c phase has a sudden change of ΔK0 of 11.45%, which exceeds the line selection criterion threshold ΔK set to 5%. n0 , so it is judged as the fault phase. The amplitude ratio mutation of the 0th harmonic of each phase other than this does not exceed the judgment threshold, so it is judged to be normal. At this time, the line selection is correct.

[0084]

[0085]

[0086] Table 3 sets the fault location on line L3 b, 25 km away from the busbar

[0087] It can be seen from Table 3 that when the fault is located on the L3 line b phase, the amplitude ratio of the 0th harmonic of the L3 line b phase changes by 6.69%, which exceeds the line selection criterion threshold ΔK set to 5%. n0 , so it is judged as the fault phase. The amplitude ratio mutation of the 0th harmonic of each phase other than this does not exceed the judgment threshold, so it is judged to be normal. At this time, the line selection is correct.

[0088] It can be seen from the table of simulation results that when a single-phase grounding fault occurs at any distance of any line in the system, only the 0th harmonic amplitude ratio mutation value ΔK0 of the fault phase will change abnormally and exceed the line selection criterion threshold ΔK n0 , while the normal phase will not undergo abnormal changes, and the correct line selection result can be obtained by fault line selection based on this.

[0089] Matlab simulation results prove the correctness of the line selection method for single-phase grounding faults in complex line distribution networks with small current grounding. This method can quickly and accurately select the fault line without installing additional equipment, ensuring the continuity and stability of power supply. It provides an efficient and economical solution for fault diagnosis and line selection of power systems, and also brings a more convenient and sustainable management method to the development of the power industry.

[0090] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them. Although the present invention is described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein by equivalents. These modifications or replacements do not make the essence of the corresponding technical solutions deviate from the technical solutions of the embodiments of the present invention. Technologies not covered by the present invention can be implemented by existing technologies.

Claims

1. A method for selecting a complex line with a small current single-phase ground fault based on the sudden change of harmonic amplitude ratio, characterized by: The steps of this method are as follows: S1. Set the sampling frequency to synchronously collect the three-phase current signals of each line of the distribution network; S2, setting a harmonic analysis period, performing harmonic analysis on each phase of the three-phase current signal collected in step S1, and extracting the amplitude data of the harmonic current of each phase within each harmonic analysis period; S3, based on the amplitude data of each phase harmonic current obtained in step S2, calculate the harmonic amplitude ratio K of the phase fundamental current corresponding to the amplitude data of different phase harmonic currents in each harmonic analysis cycle of each line. n ; S4, harmonic amplitude ratio K obtained in step S3 n , horizontally compare the harmonic amplitude ratio K of two adjacent harmonic analysis cycles on the same phase of the same line n , calculate the harmonic amplitude ratio mutation ΔK n ; S5. Determine the harmonic amplitude ratio mutation value ΔK based on the difference in harmonic characteristics between the faulty line and the non-faulty line. n The line selection criterion threshold ΔK n0 ; S6: Compare the line selection criterion threshold ΔK obtained in step S5 n0 and the 0th harmonic amplitude ratio mutation amount ΔK0 of each line obtained in step S4, and judging whether the 0th harmonic amplitude ratio mutation amount ΔK0 has abnormal changes, if not, returning to step S1; If an abnormal change occurs, it is judged that a small current grounding fault has occurred and marked as ΔK 0变 , proceed to step S7; S7. Change the 0th harmonic amplitude of the abnormal phase of the line with abnormal change to the sudden change value ΔK 0变 The vertical harmonic amplitude of the entire line is compared with the sudden change ΔK0. If ΔK 0变 Not the maximum value, return to step S1; if ΔK 0变 is the maximum value, then the abnormal phase of the line with the abnormal change is determined to be the fault phase of the fault line.

2. The complex line small current single-phase grounding fault selection method based on the harmonic amplitude ratio mutation according to claim 1 is characterized by: The three-phase current signal in step S1 is obtained through a current transformer at the voltage bus outlet.

3. The complex line small current single-phase grounding fault selection method based on harmonic amplitude ratio mutation according to claim 1 is characterized by: The sampling frequency in step S1 is set to 12.8 kHz.

4. The complex line small current single-phase grounding fault selection method based on harmonic amplitude ratio mutation according to claim 1 is characterized by: The harmonic analysis period in step S2 is 0.02s.

5. The complex line small current single-phase grounding fault selection method based on harmonic amplitude ratio mutation according to claim 1 is characterized by: The harmonic analysis in step S2 adopts the Fourier series transform method: Among them, i(t) is the current signal of one phase of the three-phase current of a certain line, ω is the angular velocity of the current of this phase under the power frequency of 50Hz, I0 is the amplitude of the zero-order harmonic current of this phase, also known as the DC component, I n It represents the amplitude of the nth harmonic current of this phase, t is the actual time, is the phase angle of the nth harmonic current of this phase, and n is the harmonic order.

6. The complex line small current single-phase grounding fault selection method based on harmonic amplitude ratio mutation according to claim 1 is characterized by: The harmonic amplitude ratio K in step S3 n The calculation method is: Among them, I1 is the fundamental current amplitude of a phase of a line in a certain harmonic analysis period, that is, the 1st harmonic current amplitude in the corresponding harmonic analysis period. The harmonic amplitude ratio can be obtained by the ratio of the nth amplitude of the harmonic current of each phase of different lines to the amplitude of the fundamental current.

7. The complex line small current single-phase grounding fault selection method based on harmonic amplitude ratio mutation according to claim 1 is characterized by: The harmonic amplitude ratio mutation amount ΔK in step S4 n The calculation method is: ΔK n =K n(k+1) -K n(k) (3) Where K n(k) and K n(k+1) They respectively represent the harmonic amplitude ratios of the phase-by-phase fundamental current corresponding to the amplitude data of the nth harmonic current of a certain phase of a certain line in the previous and next adjacent harmonic analysis periods.

8. The complex line small current single-phase grounding fault selection method based on harmonic amplitude ratio mutation according to claim 1 is characterized by: The line selection criterion threshold ΔK in step S5 n0 The determination method is as follows: for any phase of any line within a certain harmonic analysis period, the amplitude of the 0th harmonic is 5% of the sudden change ΔK0 as the line selection criterion threshold ΔK n0 .

9. The complex line small current single-phase grounding fault selection method based on harmonic amplitude ratio mutation according to claim 1 is characterized by: The device used in the line selection method includes: Current acquisition module, used to collect three-phase current signals of each line in the system; Harmonic analysis module, used to perform harmonic analysis on the three-phase current signals of each line; A line selection judgment module is used to judge the fault line according to the change of the harmonic amplitude ratio mutation amount; The result output module is used to output the judgment result of the fault line.

10. The complex line small current single-phase grounding fault selection method based on harmonic amplitude ratio mutation according to claim 9 is characterized in that: The device used in the line selection method also includes a 5G communication module, which is used to transmit the judgment results of the faulty line to a remote monitoring center.

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