Resonant grounding power grid single-phase grounding fault line selection method and device

Abstract

The invention relates to the technical field of power grid fault line selection, in particular to a resonant grounding power grid single-phase grounding fault line selection method and device. The method comprises the following steps: decomposing a zero-sequence current signal of a target power grid according to a line fault condition, and solving a maximum value in each component as an ideal partition value of a line selection criterion; fitting a polynomial curve, and performing fault domain division according to the polynomial curve and the current line zero sequence current; if all the criteria enter the fusion domain, multi-criterion fusion fault domain division is carried out, if a preset number of criteria enter the fusion domain, the credibility of the criteria is determined, and the line with the highest value, the line with only one criterion entering the fusion domain and the line with the criterion entering the effective domain are selected as fault lines; according to the invention, correct and reliable line selection in the effective domain is realized, advantage complementation is carried out on a plurality of line selection criteria in the fusion domain, the coverage of various fault conditions is improved, interference of the line selection criteria entering the failure domain is eliminated, and an accurate and reliable line selection result is obtained.

Description

Technical Field

[0001] The present invention relates to the technical field of power grid fault line selection, and in particular to a method and device for single-phase grounding fault line selection in a resonant grounding power grid. Background Art

[0002] In order to improve the reliability of power supply, most medium-voltage distribution networks are resonant grounded power grids, which compensate for the capacitive current and extinguish the arc when a single-phase grounding fault occurs in the distribution network. The generation of zero-sequence current is a unique feature of grounding faults and is usually used for fault line selection. However, due to the compensation effect of the arc suppression coil, the zero-sequence current of the fault line is much smaller than the load current, and the accuracy of directly using the zero-sequence current to judge the fault line is low. The line selection problem of single-phase grounding faults in resonant grounded power grids still needs to be solved urgently. However, the existing line selection criteria for single-phase grounding faults in resonant grounded power grids have their applicable conditions, that is, the effective domain. Changes in fault conditions will cause the line selection criteria to deviate from the effective domain and become invalid. Summary of the invention

[0003] The present invention proposes a single-phase grounding fault line selection method and device for a resonant grounding power grid, so as to solve the problem that when the existing method performs fault line selection, the line judgment criterion deviates from the effective domain due to the change of fault conditions, resulting in the failure of fault line selection.

[0004] A method for selecting a single-phase grounding fault line in a resonant grounded power grid according to the present invention comprises:

[0005] Obtain the most favorable fault conditions for line selection corresponding to each line selection criterion of the target power grid, the zero-sequence current signals when lines of different lengths are faulty, and the zero-sequence current signals of the current lines;

[0006] According to the fault condition that is most conducive to line selection, the zero-sequence current signal of the target power grid is decomposed by EEMD at the time of the fault, and the maximum value of each fault component obtained after the decomposition is obtained, which is used as the ideal partition value corresponding to each line selection criterion;

[0007] A polynomial curve is fitted according to the ideal partition value, and a line selection criterion fault domain division is performed according to the polynomial curve and the current line zero-sequence current signal, wherein the fault domain includes a valid domain, a failure domain and a fusion domain;

[0008] If all the criteria of the line are divided into the fusion domain, multiple criteria are fused and the fault domain is divided again according to the fusion result. If a predetermined number of criteria enter the fusion domain, the credibility of each criterion is determined, and the line corresponding to the highest credibility value, the line with only one criterion entering the fusion domain, and the line with the criteria entering the valid domain are selected as the faulty line.

[0009] Preferably, before obtaining the fault condition most favorable for line selection corresponding to each line selection criterion of the target power grid, determining the fault condition most favorable for line selection, the method includes:

[0010] Obtain the network topology when a single-phase grounding fault occurs in the target power grid;

[0011] According to the network topology, determining the fault grounding current, grounding point capacitance current and inductance current of the target power grid corresponding to different fault conditions;

[0012] According to the fault grounding current, the capacitive current and the inductive current, the fault condition most conducive to line selection corresponding to each line selection criterion is determined respectively.

[0013] Preferably, the method for determining the target power grid fault grounding current, grounding point capacitance current and inductance current according to the network topology includes:

[0014] The fault grounding current i is calculated using formula (1): d ;

[0015]

[0016] The grounding point capacitance current i is calculated using formula (2): c ;

[0017]

[0018] The grounding point inductor current i is calculated using formula (3): L ;

[0019]

[0020] Where: I cm is the capacitance current amplitude; I Lm is the inductor current amplitude; ω is the grid angular frequency; ω1 is the angular frequency of the transient free oscillation component; is the phase angle of the power supply voltage; r1 is the attenuation coefficient of the transient free oscillation component; t is the time constant.

[0021] Preferably, the method for performing EEMD decomposition on the zero-sequence current signal of the target power grid during a fault according to the fault condition most conducive to line selection comprises:

[0022] Add a random Gaussian white noise sequence to the zero-sequence current signal of each line;

[0023] According to the EEMD decomposition rule, the added zero-sequence current signal is decomposed into n eigenmode functions and 1 residual component by using EMD;

[0024] Repeat the adding and decomposing process to obtain all IMF components and residual components in turn;

[0025] Formula (4) is used to calculate the average values ​​of each IMF component and the residual component to obtain the fault components corresponding to each criterion of the final zero-sequence current signal:

[0026]

[0027] Where: N is the number of IMF components and residual components; c i are IMF components and residual components; m is the current IMF component or residual component.

[0028] Preferably, the method of obtaining the maximum value of each fault component obtained after decomposition and using it as the ideal partition value corresponding to each line selection criterion includes:

[0029] Performing a fast Fourier transform on each fault component of each criterion, and determining, according to the Fourier transform result, a fault component corresponding to a Fourier transform result corresponding to each criterion whose frequency is within a predetermined natural frequency range corresponding to the criterion, among the Fourier transform results corresponding to the criterion;

[0030] Adding the fault components corresponding to the frequencies within a predetermined natural frequency range to obtain a reconstructed high-frequency transient signal;

[0031] The high-frequency transient signal is smoothed by using a moving average, and the maximum value of the current signal in the smoothed high-frequency transient signal is used as the ideal partition value of the corresponding criterion.

[0032] Preferably, the method of fitting a polynomial curve according to the ideal partition values ​​comprises:

[0033] The least square method is used to fit the curve polynomial so that the sum of the squares of the distances between each discrete point and the formed fitting function is minimized;

[0034] Among them, the fifth-order polynomial is used for overhead line fitting, and in the various criteria of cable hybrid lines, the eighth-order polynomial is used for transient component and mutation component fitting, and the fifth-order polynomial is used for DC component.

[0035] Preferably, the method of fitting a polynomial curve according to the ideal partition value and performing line selection criterion fault domain division according to the polynomial curve and the current line zero-sequence current signal includes:

[0036] Obtain the current fault condition of the target distribution network;

[0037] According to the current fault condition, the current zero-sequence current signal of the target power grid is decomposed by EEMD, and the maximum value of each fault component obtained after the decomposition is obtained, which is used as the measured partition value corresponding to each line selection criterion;

[0038] Determine the ratio of the measured partition value to the corresponding polynomial curve value as the validity;

[0039] If the validity is greater than 0.8, the criterion is classified into the valid domain. If the ratio is less than or equal to 0.8 and greater than 0.2, the criterion is classified into the fusion domain. If the ratio is less than or equal to 0.2, the criterion is classified into the failure domain.

[0040] Preferably, the method for performing multi-criteria fusion includes:

[0041] Normalizing the effectiveness corresponding to the lines whose criteria enter the fusion domain;

[0042] According to the normalized validity, the BPA function m corresponding to each criterion is calculated using formula (5): B (A);

[0043]

[0044] Where: GEi is the normalized validity of a criterion on the i-th line; A is each line; rank(GEi) is the ranking of the normalized validity of a criterion on the i-th line among all normalized validity in the fusion domain;

[0045] According to the BPA function, the basic probability distribution function value of each criterion is determined, and according to the basic probability distribution function value, the fusion is performed using formula (6) to obtain a fusion result;

[0046]

[0047] in,

[0048] Where: m1(A i ) is the effectiveness of criterion 1 after normalization on line A; m2(B j ) is the validity of criterion 2 after normalization on line B; C is the body of evidence after fusion of lines AB; Φ is the uncertain part.

[0049] Preferably, the method for determining the credibility corresponding to each criterion includes:

[0050] Formula (7) is used to determine the credibility of the predetermined number of criteria entering the fusion domain;

[0051]

[0052] Where: Ei is the validity of the criterion, min(Ei) is the minimum value of the validity of all the criteria entering the fusion domain.

[0053] According to one aspect of the present invention, a single-phase grounding fault line selection device for a resonant grounded power grid is provided, comprising:

[0054] The voltage and current module is used to detect the zero-sequence current signal of the current line of the target power grid and transmit it to the control module;

[0055] A control module is used to obtain the most favorable fault conditions for line selection corresponding to each line selection criterion of the target power grid, and zero-sequence current signals when lines of different lengths are faulty; according to the fault conditions that are most favorable for line selection, the zero-sequence current signal of the target power grid when it is faulty is decomposed by EEMD, and the maximum value of each fault component obtained after decomposition is obtained, and the maximum value is used as the ideal partition value corresponding to each line selection criterion; a polynomial curve is fitted according to the ideal partition value, and the fault domain of the line selection criterion is divided according to the polynomial curve and the current line zero-sequence current signal, wherein the fault domain includes a valid domain, a failure domain and a fusion domain; if all criterion divisions of the line enter the fusion domain, multiple criterion fusion is performed, and the fault domain is divided again according to the fusion result, and if a predetermined number of criterions enter the fusion domain, the credibility corresponding to each criterion is determined, and the line corresponding to the highest credibility value, the line with only one criterion entering the fusion domain, and the line with the criterion entering the valid domain are selected as the fault line;

[0056] The display screen is used to display the final line selection result of the control module; the input and output module is connected to the control module and is used to detect the switch status of the target power grid; the power supply module is used to supply power to the control module and the display screen.

[0057] The present invention has at least the following beneficial effects:

[0058] The present invention proposes a single-phase grounding fault line selection method and device for a resonant grounded power grid, which decomposes and reconstructs a zero-sequence current signal according to a fault condition to determine a partition value; divides the fault domain according to the partition value, and performs multi-criteria fusion line selection according to the division result, thereby achieving correct and reliable line selection within the effective domain, complementing the advantages of multiple line selection criteria in the fusion domain, improving the coverage of various fault conditions, eliminating the interference of line selection criteria entering the failure domain, and obtaining accurate and reliable line selection results. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] The accompanying drawings herein are incorporated into and constitute a part of the specification. These drawings illustrate embodiments consistent with the present invention and, together with the specification, are used to explain the technical solutions of the present invention.

[0060] Figure 1A flow chart showing a method for selecting a line for a single-phase grounding fault in a resonant grounding power grid according to an embodiment of the present invention is shown;

[0061] Figure 2 A network topology diagram according to an embodiment of the present invention is shown, wherein Figure a is an overhead line simulation model, and Figure b is a cable line hybrid simulation model;

[0062] Figure 3 A fault signal waveform diagram according to an embodiment of the present invention is shown;

[0063] Figure 4 A zero-sequence current signal EEMD decomposition diagram according to an embodiment of the present invention is shown;

[0064] Figure 5 A polynomial curve diagram of transient fault quantity of overhead lines according to an embodiment of the present invention is shown;

[0065] Figure 6 A polynomial curve diagram of overhead line DC fault quantity fitting according to an embodiment of the present invention is shown;

[0066] Figure 7 A polynomial curve diagram of overhead line sudden fault quantity fitting according to an embodiment of the present invention is shown;

[0067] Figure 8 A polynomial curve diagram of transient fault quantity of hybrid cable line according to an embodiment of the present invention is shown;

[0068] Fig. 9 A polynomial curve diagram of the DC fault quantity of the cable hybrid line according to an embodiment of the present invention is shown;

[0069] Fig.10 A polynomial curve fitting of the sudden fault quantity of a cable hybrid line according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0070] Various exemplary embodiments, features and aspects of the present invention will be described in detail below with reference to the accompanying drawings. The same reference numerals in the accompanying drawings represent elements with the same or similar functions. Although various aspects of the embodiments are shown in the accompanying drawings, the drawings are not necessarily drawn to scale unless otherwise specified.

[0071] The word “exemplary” is used exclusively herein to mean “serving as an example, example, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments.

[0072] The term "and / or" herein is only a description of the association relationship of the associated objects, indicating that there may be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the term "at least one" herein represents any combination of at least two of any one or more of a plurality of. For example, including at least one of A, B, and C can represent including any one or more elements selected from the set consisting of A, B, and C.

[0073] In addition, in order to better illustrate the present invention, numerous specific details are provided in the following specific embodiments. It should be understood by those skilled in the art that the present invention can be implemented without certain specific details. In some examples, methods, means, components and circuits well known to those skilled in the art are not described in detail in order to highlight the subject matter of the present invention.

[0074] Figure 1 A flow chart showing a method for selecting a line for a single-phase grounding fault in a resonant grounding power grid according to an embodiment of the present invention is shown; Figure 2 A network topology diagram showing an embodiment according to an embodiment of the present invention; Figure 3 A fault signal waveform diagram according to an embodiment of the present invention is shown; Figure 4 A zero-sequence current signal EEMD decomposition diagram according to an embodiment of the present invention is shown; Figure 5 A polynomial curve diagram of transient fault quantity of overhead lines according to an embodiment of the present invention is shown; Figure 6 A polynomial curve diagram of overhead line DC fault quantity fitting according to an embodiment of the present invention is shown; Figure 7 A polynomial curve diagram of overhead line sudden fault quantity fitting according to an embodiment of the present invention is shown; Figure 8 A polynomial curve diagram of transient fault quantity of hybrid cable line according to an embodiment of the present invention is shown; Fig. 9 A polynomial curve diagram of the DC fault quantity of the cable hybrid line according to an embodiment of the present invention is shown; Fig.10 The following is a polynomial curve diagram showing the sudden fault quantity of the cable hybrid line according to an embodiment of the present invention. Figure 1-10As shown, a method for selecting a line for a single-phase grounding fault in a resonant grounding power grid comprises: step S01: obtaining the most favorable fault condition for line selection corresponding to each line selection criterion of the target power grid, zero-sequence current signals when lines of different lengths are faulty, and zero-sequence current signals of the current line; step S02: performing EEMD decomposition on the zero-sequence current signal of the target power grid when it is faulty according to the fault condition that is most favorable for line selection, obtaining the maximum value of each fault component obtained after decomposition, and taking it as the ideal partition value corresponding to each line selection criterion; step S03: fitting a polynomial curve according to the ideal partition value, dividing the fault domain of the line selection criterion according to the polynomial curve and the current line zero-sequence current signal, wherein the fault domain includes an effective domain, an ineffective domain, and a fusion domain; step S04: if all criterion divisions of the line enter the fusion domain, multi-criteria fusion is performed, and the fault domain is divided again according to the fusion result, if a predetermined number of criteria enter the fusion domain, the credibility corresponding to each criterion is determined, and the line corresponding to the highest credibility value, the line with only one criterion entering the fusion domain, and the line with the criterion entering the effective domain are selected as the fault line.

[0075] The method for selecting a single-phase grounding fault line in a resonant grounded power grid provided by an embodiment of the present invention specifically comprises the following steps:

[0076] Step S01: Obtain the most favorable fault condition for line selection corresponding to each line selection criterion of the target power grid, the zero-sequence current signal when the lines of different lengths are faulty, and the zero-sequence current signal of the current line.

[0077] In the present invention, before obtaining the fault condition that is most favorable for line selection corresponding to each line selection criterion of the target power grid, determining the fault condition that is most favorable for line selection, the method includes: obtaining the network topology when a single-phase grounding fault occurs in the target power grid; determining the fault grounding current, grounding point capacitance current and inductance current of the target power grid corresponding to different fault conditions according to the network topology; and determining the fault condition that is most favorable for line selection corresponding to each line selection criterion according to the fault grounding current, capacitance current and inductance current.

[0078] In the present invention, the method for determining the target grid fault grounding current, grounding point capacitance current and inductance current according to the network topology includes: using formula (1) to calculate the fault grounding current i d ;

[0079]

[0080] The grounding point capacitance current i is calculated using formula (2): c ;

[0081]

[0082] The grounding point inductor current i is calculated using formula (3): L ;

[0083]

[0084] Where: I cm is the capacitance current amplitude; I Lm is the inductor current amplitude; ω is the grid angular frequency; ω1 is the angular frequency of the transient free oscillation component; is the phase angle of the power supply voltage; r1 is the attenuation coefficient of the transient free oscillation component; t is the time constant.

[0085] In the embodiment of the present invention, the network topology when a single-phase grounding fault occurs in the resonant grounding power grid is obtained according to the known power grid structure of the target power grid.

[0086] According to the network topology, determine the grounding current expression of the resonant grounding grid when a single-phase grounding fault occurs. The network topology diagram is as follows Figure 2 As shown, the zero sequence signal is Figure 3 When a single-phase ground fault occurs in a resonant grounded grid, the fault ground current is composed of the superposition of the capacitive current and the inductive current. The fault ground current, i.e., the zero-sequence current signal i d The calculation formula is shown in formula (1).

[0087] From formula (1), we can get the grounding point capacitance current i c The calculation formula is shown in formula (2).

[0088] From formula (1), we can get the grounding point inductor current i L The calculation formula is shown in formula (3).

[0089] According to the fault grounding current expression, the fault components such as fault transient component, fault mutation component and fault DC component are obtained to form the line selection criterion.

[0090] From formula (1), it can be seen that the arc suppression coil presents high impedance to high-frequency transient components, the transient components on the fault line and the non-fault line have opposite polarities, and the amplitude of the transient component on the fault line is approximately the sum of the amplitudes of the non-fault line. Therefore, the transient component can be used as a characteristic quantity for line selection.

[0091] From formula (2), we can see that the current in the arc suppression coil is mainly composed of DC component and steady-state component. Therefore, when a fault occurs, the DC component of the fault line is all provided by the arc suppression coil, and its value is equal to the DC component of the coil, but opposite in direction; while the non-fault line has no DC component. Therefore, the DC component can be used as a characteristic quantity for line selection.

[0092] It can be seen from formula (3) that at the beginning of the fault, the current in the arc suppression coil is almost 0. Under the action of the fault grounding point, in the first half-wave cycle of the fault, the mutation component on the fault line is approximately the opposite of the comprehensive value on the non-fault line. Therefore, the mutation component can also be used as a characteristic quantity for line selection.

[0093] Due to the compensation characteristics of the arc suppression coil, the line selection methods based on steady-state quantities such as zero-sequence current magnitude and phase are invalid. Therefore, the present invention analyzes the line selection criteria formed by high-frequency transient components, low-frequency DC components and mutation components to seek their effective domain value range.

[0094] According to the formed line selection criteria, the fault characteristics are made most obvious by changing the fault conditions, that is, the fault components corresponding to each line selection criterion have a maximum value under a specific fault condition, and the state of each line selection criterion that is most conducive to line selection is obtained;

[0095] 1. The transient quantity (component) line selection criterion is most conducive to the fault conditions of line selection.

[0096] From formula (2), it can be seen that when a single-phase grounding fault occurs at the voltage peak, the amplitude of the high-frequency transient component reaches the maximum value; the closer the fault location is to the busbar, the higher the natural frequency, and the larger the amplitude of the high-frequency transient component; the smaller the transition resistance, the larger the amplitude of the high-frequency transient component. The conditions are: (1) the initial phase angle of the fault is 90 degrees; (2) the fault location occurs 1 km away from the busbar; (3) the fault type is a metallic grounding fault. At this time, the transient quantity criterion fault characteristics are most obvious.

[0097] 2. The DC line selection criterion is most conducive to the fault conditions of line selection.

[0098] It can be seen from formula (3) that the low-frequency DC amplitude is also related to the voltage phase angle. When the voltage is 0, the amplitude is the largest. The closer the fault location is to the busbar, the larger the low-frequency DC component amplitude is. The smaller the transition resistance is, the larger the low-frequency DC component amplitude is. The conditions are: (1) the initial phase angle of the fault is 0 degrees; (2) the fault location occurs 1 km away from the busbar; (3) the fault type is a metallic grounding fault. At this time, the DC quantity criterion fault characteristics are most obvious.

[0099] 3. The sudden change line selection criterion is most conducive to the fault conditions of line selection.

[0100] From formula (1), it can be seen that when a single-phase grounding fault occurs at the voltage peak, the amplitude of the mutation component reaches the maximum value; the closer the fault location is to the busbar, the larger the amplitude of the mutation component; the smaller the transition resistance, the larger the amplitude of the mutation component. The conditions are: (1) the initial phase angle of the fault is 90 degrees; (2) the fault location occurs 1 km away from the busbar; (3) the fault type is a metallic grounding fault. At this time, the fault characteristics of the mutation quantity criterion are most obvious.

[0101] Step S02: According to the fault condition most conducive to line selection, the zero-sequence current signal of the target power grid is decomposed by EEMD, and the maximum value of each fault component obtained after decomposition is obtained, which is used as the ideal partition value corresponding to each line selection criterion.

[0102] In the present invention, the method for performing EEMD decomposition on the zero-sequence current signal of the target power grid during a fault according to the fault condition that is most conducive to line selection comprises: adding a random Gaussian white noise sequence to the zero-sequence current signal of each line; decomposing the added zero-sequence current signal into n intrinsic mode functions and 1 residual component by EMD according to the EEMD decomposition rule; repeating the adding and decomposition process to obtain all IMF components and residual components in turn; calculating the average values ​​of each IMF component and the residual component by using formula (4) to obtain each fault component corresponding to each criterion of the final zero-sequence current signal

[0103] Where: N is the number of IMF components and residual components; c i are IMF components and residual components; m is the current IMF component or residual component.

[0104] In an embodiment of the present invention, when a single-phase grounding fault occurs in a resonant grounded power grid, the fault transient component, fault DC component, IMF component and residual component of the fault transient component under the fault conditions with the most obvious fault characteristics and under the measured fault conditions are extracted through EEMD, laying the foundation for the next step of reconstructing the fault component extraction partition value.

[0105] Ensemble empirical mode decomposition (EEMD) is a noise-assisted data analysis method. This method can effectively explore the implicit components in the signal, avoid the selection of basis functions in wavelet transform, and adaptively extract fault features in zero-sequence current, which is very practical.

[0106] The process of EEMD decomposing zero-sequence current signal is as follows:

[0107] 1. Add random Gaussian white noise n to the zero-sequence current signal x(t) of each line m (t) sequence, we get

[0108] x m (t) = x(t) + n m (t);

[0109] 2. Decompose the EMD into n intrinsic mode functions (IMF components) and 1 residual component;

[0110] 3. Repeat steps 1 and 2 to obtain the IMF component and the residual component respectively;

[0111] 4. Use formula (4) to calculate the average value of the IMF component and the residual component to obtain the final components of the zero-sequence current signal.

[0112] In the present invention, the method of obtaining the maximum value of each fault component obtained after decomposition and using it as the ideal partition value corresponding to each line selection criterion includes: performing fast Fourier transform on each fault component of each criterion, and determining, based on the Fourier transform result, the fault component corresponding to the Fourier transform result corresponding to each criterion, whose frequency is within the predetermined natural frequency range corresponding to the criterion; adding the fault components corresponding to the frequencies within the predetermined natural frequency range to obtain a reconstructed high-frequency transient signal; smoothing the high-frequency transient signal using a moving average, and using the maximum value of the current signal in the smoothed high-frequency transient signal as the ideal partition value of the corresponding criterion.

[0113] In the embodiment of the present invention, the maximum values ​​of the fault components at different line lengths are obtained as partition values.

[0114] The natural frequency of high-frequency transient current is generally 300Hz-3000Hz, with a short duration, and it usually decays within 0.5-1 cycles; low-frequency DC lasts for a long time, and usually decays within 2-5 cycles, and has a low frequency. The IMF after EEMD decomposition is subjected to fast Fourier transform, and it can be obtained that the IMF components with a frequency below 10HZ are mainly concentrated in IMF10 and the remaining components; the mutation components are concentrated in the first half-wave cycle of the zero-sequence current when the fault occurs. The mutation components have the characteristics of large amplitude and fast change, so EEMD is used to extract the highest frequency IMF1 component, find the maximum and minimum values ​​in the component, and use the extreme value difference as the mutation component to distinguish between faulty lines and non-faulty lines; for the transmission of cable hybrid lines, due to the large cable capacitance, the highest frequency IMF1 component obtained by decomposition is similar to the faulty line, while the high-frequency IMF4 signal is not affected, so the IMF4 component is extracted from the cable hybrid line as the mutation amount. Taking the transient fault component as an example, the above decomposition obtained Figure 4 By performing fast Fourier transform on each transient IMF component, it can be found that the IMF components with frequencies (Fourier transform results) above 300Hz are mainly concentrated in the IMF1-IMF4 components. The IMF1-IMF4 components are added together to reconstruct a high-frequency transient signal.

[0115] However, the signal is too rough and may be discontinuous. Therefore, the present invention performs moving average smoothing on the reconstructed signal, as shown in formula (8). If the measured signals (zero-sequence current signals after the addition of IMF1-IMF4) are x1, x2, x3, x4, and x5, and the three are smoothed as a group, the moving average is:

[0116]

[0117] The arithmetic mean values ​​of the original high-frequency transient signals after moving average are reconstructed into high-frequency transient signals. The maximum value of the high-frequency transient signal under the line length at this time (the maximum value on the smoothed curve) is extracted, that is, the ideal partition value of the transient component line selection criterion under the corresponding line length, and the partition values ​​of all criteria are obtained by analogy. By continuing to change the line length, the ideal partition values ​​corresponding to different line selection criteria and different lengths can be obtained.

[0118] Step S03: fitting a polynomial curve according to the ideal partition value, and performing line selection criterion fault domain division according to the polynomial curve and the current line zero-sequence current signal, wherein the fault domain includes a valid domain, a failure domain and a fusion domain.

[0119] In the present invention, the method of fitting a polynomial curve according to the ideal partition value includes: using the least squares method to fit the curve polynomial so that the sum of the squares of the distances between each discrete point and the formed fitting function is minimized; wherein, a fifth-order polynomial is used for overhead line fitting, and in the various criteria of cable hybrid lines, an eighth-order polynomial is used for fitting transient components and mutation components, and a fifth-order polynomial is used for DC components.

[0120] In an embodiment of the present invention, the least squares method is used to fit the curve polynomial, and the fitting criterion is to minimize the sum of the squares of the distances between each discrete point and the formed fitting function. The least squares method is used to fit the discrete points to form a curve, where x corresponds to the line length, y corresponds to the partition value of each component, and n represents the order to be fitted. The order of fitting determines the accuracy of the function polynomial, but too low a fitting order will lead to an increase in the error of the fitting curve, and too high a fitting order will lead to data redundancy and complex calculations. The overhead line fitting curve uses a 5th-order polynomial, that is, n=5; the transient and sudden changes of the cable hybrid line use an 8th-order polynomial, and the DC value uses a 5th-order polynomial.

[0121] Among them, the ideal partition value fitting polynomial of overhead line transient quantity (component) is:

[0122] y1=-0.00033335x 5 +0.015495x 4 -0.22073x 3 +0.45884x 2 +15.903x-46.863;

[0123] The fitting polynomial of the ideal partition value of the transient quantity of the cable hybrid line is:

[0124] y2=-0.00006724x 5 +0.0044177x 4-0.11334x 3 +1.4142x 2 -8.1862x+19.585;

[0125] The ideal partition value fitting polynomial of the DC value of overhead lines is:

[0126] y3=-0.0049078x 5 +0.34122x 4 -8.9327x 3 +108.27x 2 -597.51x+1305.3;

[0127] The fitting polynomial of the ideal partition value of the DC quantity of the cable hybrid line is:

[0128] y4=-0.00041113633x 8 +0.043551053x 7 -1.9774446x 6 +50.191916x 5 -777.73101x 4 +7521.8106x 3 -44281.193x 2 +144955.2x-201750.88;

[0129] Fitting polynomial of the partition value of the sudden change of overhead line:

[0130] y5=-0.0010084x 5 +0.067147x 4 -1.7054x 3 +20.117x 2 -103.35x+193.84;

[0131] The fitting polynomial of the ideal partition value of the cable hybrid line mutation is:

[0132] y6=-0.00026956327x 8 +0.028049838x 7 -1.2483995x 6 +30.995761x 5 -469.0093x 4 +4425.055x 3 -25410.104x 2 +81202.653x-110574.51;

[0133] The polynomial curves of the overhead line criterion are as follows: Figure 5-7 As shown in the figure, the polynomial curves of the cable hybrid line are fitted as follows: Figure 8-10 shown.

[0134] In the present invention, the method of fitting a polynomial curve according to the ideal partition value, and dividing the fault domain of the line selection criterion according to the polynomial curve and the current line zero-sequence current signal, includes: obtaining the current fault condition of the target distribution network; performing EEMD decomposition on the current zero-sequence current signal of the target power grid according to the current fault condition, and obtaining the maximum value of each fault component obtained after the decomposition, and using it as the measured partition value corresponding to each line selection criterion; determining the ratio of the measured partition value to the corresponding polynomial curve value as the validity; if the validity is greater than 0.8, the criterion is divided into the valid domain, if the ratio is less than or equal to 0.8 and greater than 0.2, the criterion is divided into the fusion domain, and if the ratio is less than or equal to 0.2, the criterion is divided into the failure domain.

[0135] In the embodiment of the present invention, according to the obtained polynomial curve, the line selection criterion fault domain is divided into a valid domain, a failure domain and a fusion domain.

[0136] The zero-sequence current signal measured on the current line is decomposed by EEMD according to the current grid fault conditions to obtain the corresponding IMF components of each line selection criterion (DC, transient, and mutation), and the maximum value of each component of each criterion is taken as the corresponding measured partition value.

[0137] When the measured partition value is used to divide the fault domain, if the measured partition value of the line selection criterion enters the effective domain, the criterion should be able to select the line correctly 100%, which means that the fault feature quantity used by the criterion is the most obvious in the fault line and has the greatest difference from the non-fault line. When measuring the partition value, the content of the fault feature quantity in the fault line is much greater than that in the non-fault line, but as the fault conditions change, the content of the feature quantity continues to decrease. In order to make a conservative judgment, the proportion of the maximum value of the measured feature quantity (measured partition value) of a certain line in the ideal partition value (polynomial fitting curve value) is defined as the validity; when the validity is above 0.8, the line selection criterion corresponding to this feature quantity (measured partition value) enters the effective domain; when the validity is between 0.8 and 0.2, the corresponding line selection criterion enters the fusion domain; when the validity is less than 0.2, the corresponding line selection criterion enters the failure domain. The specific calculation formulas for each fault domain division are shown in equations (9) to (11).

[0138] Valid domain division:

[0139]

[0140] Failure domain division:

[0141]

[0142] Fusion domain division:

[0143]

[0144] Where: zm(k) is the measured partition value; zy(k) is the polynomial curve value; k is the line number; i is the line selection criterion used.

[0145] Step S04: If all the criteria of the line are divided into the fusion domain, multiple criteria are fused and the fault domain is divided again according to the fusion result. If a predetermined number of criteria enter the fusion domain, the credibility of each criterion is determined, and the line corresponding to the highest credibility value, the line with only one criterion entering the fusion domain, and the line with the criteria entering the valid domain are selected as the faulty line.

[0146] In an embodiment of the present invention, according to the fault domain division result of the line selection criteria, if one or more line selection criteria of a line enter the valid domain, the line is determined to be a faulty line; if all the line selection criteria of a line enter the failure domain, the line is eliminated; if all three line selection criteria of a line enter the fusion domain, the line is selected after multi-criteria fusion; if two enter the fusion domain and one enters the failure domain, the line is selected by determining the credibility; if one enters the fusion domain and two enter the failure domain, the line is determined to be a faulty line.

[0147] In the present invention, the method for fusing multiple criteria includes: normalizing the effectiveness corresponding to the lines of all criteria entering the fusion domain;

[0148] According to the normalized validity, the BPA function m corresponding to each criterion is calculated using formula (5): B (A);

[0149]

[0150] Where: GEi is the normalized validity of a criterion on the i-th line; A is each line; rank(GEi) is the ranking of the normalized validity of a criterion on the i-th line among all normalized validity in the fusion domain;

[0151] According to the BPA function, the basic probability distribution function value of each criterion is determined, and according to the basic probability distribution function value, the fusion is performed using formula (6) to obtain a fusion result;

[0152]

[0153] in,

[0154] Where: m1(A i) is the effectiveness of criterion 1 after normalization on line A; m2(B j ) is the validity of criterion 2 after normalization on line B; C is the body of evidence after fusion of lines AB; Φ is the uncertain part.

[0155] In an embodiment of the present invention, evidence theory is used to perform multi-criteria fusion. A decision problem is assumed. The set of all decision results for the problem is called a recognition framework Θ. Evidence theory is a method for determining the degree to which a priori unknown object belongs to a subset A in Θ.

[0156] Given the recognition frame Θ, in the power set 2 Θ Define a function m(·) on the Θ →[0,1], and satisfies:

[0157]

[0158] m(·) is called the basic probability assignment function (BPA), which indicates the degree of confidence in the conclusion. m(Θ) indicates the degree of confidence in the assignment to uncertainty, and each BPA function is called the body of evidence Θ on the recognition framework.

[0159] The core of evidence theory is the combination of evidence bodies. Dempster combination rule is used to synthesize each evidence body. The rule definition is shown in formula (6).

[0160] As for the line selection problem, in order to construct the BPA function, the effectiveness (proportion) of each criterion on each line is first normalized, as shown in the following formula (12).

[0161]

[0162] Where: Ei is the effectiveness of each line criterion; GEi is the normalized effectiveness,

[0163] Since there is no uncertain part m(θ), it is not enough to form the BPA function. Therefore, the failure domain method is adopted. When GEi is less than 0.1 and its ranking in the normalized effectiveness of all criteria in the fusion domain is not in the top 3, it is considered not to be a faulty line, and its basic credibility is configured as 0. Then the BPA function can be constructed into two cases, as shown in formula (5), that is, the normalized effectiveness of the top 3 and greater than 0.1 is taken, and the rest are taken as 0.

[0164] The remaining uncertain part when generating the BPA function can be expressed as:

[0165]

[0166] The BPA function is written in a unified form:

[0167]

[0168] The partition value algorithm is used when constructing transient quantity, sudden change quantity and DC quantity. Each criterion corresponds to a BPA function, that is, the transient quantity criterion corresponds to m1(k), the DC quantity criterion corresponds to m2(k), and the sudden change quantity criterion corresponds to m3(k). The basic probability distribution of the three line selection criteria is obtained as follows:

[0169] m1(l1), m1(l2), …, m1(l n ), m1(Θ) (13);

[0170] m2(l1), m2(l2), ..., m2(l n ), m2(Θ) (14);

[0171] m3(l1), m3(l2), ..., m3(l n ), m3(Θ) (15);

[0172] The basic probability distribution function values ​​of each line obtained by equations (13) to (15) are fused using the evidence theory synthesis rule, i.e., equation (6), to obtain the fusion result m(C). The fault domain is divided again based on the fusion result to obtain the final line selection result.

[0173] In the present invention, the method for determining the credibility corresponding to each criterion includes: using formula (7) to determine the credibility corresponding to a predetermined number of criteria entering the fusion domain;

[0174]

[0175] Where: Ei is the validity of the criterion, min(Ei) is the minimum value of the validity of all the criteria entering the fusion domain.

[0176] In the embodiment of the present invention, in order to further improve the reliability of line selection by fusion of multiple criteria, the following rules 1-3 are defined according to the number of line selection criteria entering the fusion domain.

[0177] 1. If there are three line selection criteria for a line to enter the fusion domain, that is, all the criteria enter the fusion domain, then the line is defined as a Class I line; according to the validity of the criteria on each line, the evidence theory is used for fusion, that is, after the above multi-criteria fusion, a reliable line selection result is given.

[0178] 2. If the line selection criteria for the line entering the fusion domain are the predetermined number, i.e., 2, and the remaining criteria all enter the failure domain, the line is defined as a Class II line; taking line 1 as an example, determine the difference between the normalized effectiveness GEi corresponding to the criterion of line 1 and the minimum effectiveness in the criteria of the remaining Class II lines in the fusion domain, determine the share (proportion) of the difference in the normalized effectiveness GEi of line 1, and use the share as the credibility; by analogy, determine the credibility corresponding to each criterion of all Class II lines in the fusion domain, and select the line corresponding to the highest value among all credibility as the unreliable line selection result, i.e., the faulty line. The credibility calculation formula (7) is shown below.

[0179] 3. If the line selection criterion for the line to enter the fusion domain is one, and all other criteria enter the failure domain, the line is judged to be an unreliable line selection result, that is, a faulty line.

[0180] When the number of criteria required for fusion is less than 3, the fusion result is not completely reliable. Therefore, this paper adopts the concept of credibility. The higher the credibility, the greater the difference in credibility between the lines, and the greater the probability of failure of the line.

[0181] In an embodiment of the present invention, a control variable method is adopted to perform single-phase grounding fault simulation on a resonant grounded power grid when only the transition resistance, the initial fault phase angle, and the fault position change, extract the effectiveness of each line selection criterion on each line, divide the domain value interval entered by the line selection criterion, and verify the correctness of the proposed fault line selection algorithm.

[0182] Build a 10kV distribution network simulation model such as Figure 2 The power grid is equipped with a 110kV generator, the main transformer is a type connection, the ratio is 110 / 10kV, the Z-type grounding transformer ratio is 10 / 0.4kV, and the arc suppression coil L adopts 10% overcompensation.

[0183] Overhead line simulation model Figure 2 As shown in Figure (a), lines 1-4 are all overhead lines. The hybrid simulation model of overhead lines and cable lines is shown in Figure 2 As shown in Figure (b), lines 1-3 are overhead lines, line 4 is a combination of overhead lines and cable lines, and line 5 is a cable line. The line parameters are shown in Table 1.

[0184] Table 1: Line parameter table

[0185]

[0186] Assume that the line length is 15 km, and a single-phase grounding fault occurs in phase A of line 2, where the fault location occurs 1 km away from the busbar, the initial phase angle of the fault is 90°, and the fault transition resistance is set to 0.001Ω, 10Ω, and 100Ω respectively. The fault domain value intervals of each selection criterion for overhead lines are shown in Table 2.

[0187] Table 2: Overhead line selection criteria for entering the fault domain value interval table

[0188]

[0189] The fault conditions are the same as those of overhead lines. The line selection criteria of cable hybrid lines enter the fault domain value range as shown in Table 3.

[0190] Table 3: Cable hybrid line selection criteria for entering the fault domain value interval table

[0191]

[0192] It can be understood that the above-mentioned various method embodiments mentioned in the present invention can be combined with each other to form a combined embodiment without violating the principle logic. Due to space limitations, the present invention will not go into details.

[0193] The execution subject of the resonant grounded power grid single-phase grounding fault line selection method may be a resonant grounded power grid single-phase grounding fault line selection device, for example, the resonant grounded power grid single-phase grounding fault line selection method may be executed by a terminal device or a server or other processing device, wherein the terminal device may be a user equipment (UE), a mobile device, a user terminal, a terminal, a cellular phone, a cordless phone, a personal digital assistant (PDA), a handheld device, a computing device, a vehicle-mounted device, a wearable device, etc. In some possible implementations, the resonant grounded power grid single-phase grounding fault line selection method may be implemented by a processor calling a computer-readable instruction stored in a memory.

[0194] Those skilled in the art will appreciate that, in the above method of specific implementation, the order in which the steps are written does not imply a strict execution order and does not constitute any limitation on the implementation process. The specific execution order of the steps should be determined by their functions and possible internal logic.

[0195] In the present invention, a single-phase grounding fault line selection device for a resonant grounding power grid is also provided, comprising: a voltage and current module, used to detect the zero-sequence current signal of the current line of the target power grid and transmit it to the control module; the control module is used to obtain the most favorable fault condition for line selection corresponding to each line selection criterion of the target power grid, and the zero-sequence current signal when the line has faults of different lengths; according to the fault condition that is most favorable for line selection, the zero-sequence current signal of the target power grid when it is faulty is subjected to EEMD decomposition, and the maximum value of each fault component obtained after the decomposition is obtained, which is used as the ideal partition value corresponding to each line selection criterion; a polynomial curve is fitted according to the ideal partition value, and the polynomial curve and the current are calculated according to the polynomial curve. The zero-sequence current signal of the previous line is used to divide the fault domain of the line selection criterion, wherein the fault domain includes a valid domain, a failure domain and a fusion domain; if all the criterion divisions of the line enter the fusion domain, multiple criterion fusion is performed, and the fault domain is divided again according to the fusion result. If a predetermined number of criteria enter the fusion domain, the credibility corresponding to each criterion is determined, and the line corresponding to the highest credibility value and the line with only one criterion entering the fusion domain and the line with the criterion entering the valid domain are selected as the fault line; a display screen is used to display the final line selection result of the control module; an input and output module is connected to the control module and is used to detect the switch status of the target power grid; a power supply module is used to supply power to the control module and the display screen.

[0196] One or more programs are stored in the control module and configured to be executed by one or more processors, and the one or more programs include instructions for executing the above method.

[0197] In some embodiments, the functions of the apparatus provided in an embodiment of the present invention or the modules and devices included therein can be used to execute the method described in the above method embodiment. The specific implementation thereof can refer to the description of the above method embodiment, and for the sake of brevity, it will not be repeated here.

[0198] Compared with the prior art, the method of the present invention has at least the following beneficial effects: the effective domain value of the line selection criterion is divided according to the obvious degree of the fault characteristic quantity. First, the fault characteristic quantity contained in the zero-sequence current signal is analyzed; secondly, the fault characteristic of a specific line length is changed by simulation to make the fault characteristic most obvious, and the maximum value of each fault characteristic quantity is obtained by EEMD decomposition and reconstruction as the partition value; then the partition value under different line lengths is calculated by changing the line length, and the least squares method is used to fit the polynomial curve; finally, under the actual fault condition, the maximum value of the fault characteristic quantity of the corresponding line length is measured. The proportion of the partition value is defined as the effectiveness, and the domain value interval of the line selection criterion is divided according to the size of the effectiveness. When the effectiveness of the characteristic quantity is greater than 80%, the line selection criterion enters the effective domain, and the line is directly selected using this criterion. When the effectiveness is less than 20%, the line selection criterion enters the failure domain and the criterion is discarded. When the effectiveness is between 20% and 80%, the DS evidence theory and fusion rules are used to select the line, and the credibility is given. By dividing the domain value interval, the line selection criteria in the effective domain can select the line correctly and reliably, and the multiple line selection criteria in the fusion domain can complement each other's advantages to improve the coverage of various fault conditions; and the interference of the line selection criteria entering the failure domain can be eliminated to form a reliable multi-criteria fusion line selection method.

[0199] The present invention takes the obviousness of fault characteristic quantity as the core, regards the change of fault condition as the unknown parameter, divides the effective domain, fusion domain and failure domain interval of line selection criterion, and outputs the line selection criterion entering each fault domain interval. The line selection accuracy is high, is not affected by fault conditions and network parameters, and has strong robustness and practicality.

[0200] The embodiments of the present invention have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The selection of terms used herein is intended to best explain the principles of the embodiments, practical applications, or technical improvements in the market, or to enable other persons of ordinary skill in the art to understand the embodiments disclosed herein.

Claims

1. A method for selecting a single-phase grounding fault line in a resonant grounding power grid, characterized in that: include: Obtain the most favorable fault conditions for line selection corresponding to each line selection criterion of the target power grid, the zero-sequence current signals when lines of different lengths are faulty, and the zero-sequence current signals of the current lines; According to the fault condition that is most conducive to line selection, the zero-sequence current signal of the target power grid is decomposed by EEMD at the time of the fault, and the maximum value of each fault component obtained after the decomposition is obtained, which is used as the ideal partition value corresponding to each line selection criterion; A polynomial curve is fitted according to the ideal partition value, and a line selection criterion fault domain division is performed according to the polynomial curve and the current line zero-sequence current signal, wherein the fault domain includes a valid domain, a failure domain and a fusion domain; If all the criteria of the line are divided into the fusion domain, multiple criteria are fused and the fault domain is divided again according to the fusion result. If a predetermined number of criteria enter the fusion domain, the credibility of each criterion is determined, and the line corresponding to the highest credibility value, the line with only one criterion entering the fusion domain, and the line with the criteria entering the valid domain are selected as the faulty line.

2. The method for selecting a single-phase grounding fault line in a resonant grounded power grid according to claim 1, characterized in that: Before obtaining the most favorable fault condition for line selection corresponding to each line selection criterion of the target power grid, determining the most favorable fault condition for line selection, the method includes: Obtain the network topology when a single-phase grounding fault occurs in the target power grid; According to the network topology, determining the fault grounding current, grounding point capacitance current and inductance current of the target power grid corresponding to different fault conditions; According to the fault grounding current, the capacitive current and the inductive current, the fault condition most conducive to line selection corresponding to each line selection criterion is determined respectively.

3. The method for selecting a single-phase grounding fault line in a resonant grounded power grid according to claim 2, characterized in that: The method for determining the target power grid fault grounding current, grounding point capacitance current and inductance current according to the network topology includes: The fault grounding current i is calculated using formula (1): d ; The grounding point capacitance current i is calculated using formula (2): c ; The grounding point inductor current i is calculated using formula (3): L ; Where: I cm is the capacitance current amplitude; I Lm is the inductor current amplitude; ω is the grid angular frequency; ω1 is the angular frequency of the transient free oscillation component; is the phase angle of the power supply voltage; r1 is the attenuation coefficient of the transient free oscillation component; t is the time constant.

4. The method for selecting a single-phase grounding fault line in a resonant grounded power grid according to claim 1, characterized in that: The method for performing EEMD decomposition on the zero-sequence current signal of the target power grid during the fault according to the fault condition that is most conducive to line selection comprises: Add a random Gaussian white noise sequence to the zero-sequence current signal of each line; According to the EEMD decomposition rule, the added zero-sequence current signal is decomposed into n eigenmode functions and 1 residual component by using EMD; Repeat the adding and decomposing process to obtain all IMF components and residual components in turn; Formula (4) is used to calculate the average values ​​of each IMF component and the residual component to obtain the fault components corresponding to each criterion of the final zero-sequence current signal: Where: N is the number of IMF components and residual components; c i are IMF components and residual components; m is the current IMF component or residual component.

5. The method for selecting a single-phase grounding fault line in a resonant grounding power grid according to claim 1, characterized in that: The method of obtaining the maximum value of each fault component obtained after decomposition and using it as the ideal partition value corresponding to each line selection criterion includes: Performing a fast Fourier transform on each fault component of each criterion, and determining, according to the Fourier transform result, a fault component corresponding to the Fourier transform result corresponding to each criterion, whose frequency is within a predetermined natural frequency range corresponding to the criterion; Adding the fault components corresponding to the frequencies within a predetermined natural frequency range to obtain a reconstructed high-frequency transient signal; The high-frequency transient signal is smoothed by using a moving average, and the maximum value of the current signal in the smoothed high-frequency transient signal is used as the ideal partition value of the corresponding criterion.

6. The method for selecting a single-phase grounding fault line in a resonant grounding power grid according to claim 1, characterized in that: The method of fitting a polynomial curve according to the ideal partition value comprises: The least square method is used to fit the curve polynomial so that the sum of the squares of the distances between each discrete point and the formed fitting function is minimized; Among them, the fifth-order polynomial is used for overhead line fitting, and in the various criteria of cable hybrid lines, the eighth-order polynomial is used for transient component and mutation component fitting, and the fifth-order polynomial is used for DC component.

7. The method for selecting a single-phase grounding fault line in a resonant grounded power grid according to any one of claims 1 to 6, characterized in that: The method of fitting a polynomial curve according to the ideal partition value and performing line selection criterion fault domain division according to the polynomial curve and the current line zero-sequence current signal includes: Obtain the current fault condition of the target distribution network; According to the current fault condition, the current zero-sequence current signal of the target power grid is decomposed by EEMD, and the maximum value of each fault component obtained after the decomposition is obtained, which is used as the measured partition value corresponding to each line selection criterion; Determine the ratio of the measured partition value to the corresponding polynomial curve value as the validity; If the validity is greater than 0.8, the criterion is classified into the valid domain. If the ratio is less than or equal to 0.8 and greater than 0.2, the criterion is classified into the fusion domain. If the ratio is less than or equal to 0.2, the criterion is classified into the failure domain.

8. The method for selecting a single-phase grounding fault line in a resonant grounded power grid according to claim 7, characterized in that: The method for performing multi-criteria fusion comprises: Normalizing the effectiveness corresponding to the lines whose criteria enter the fusion domain; According to the normalized validity, the BPA function m corresponding to each criterion is calculated using formula (5): B (A); Where: GEi is the normalized validity of a criterion on the i-th line; A is each line; rank(GEi) is the ranking of the normalized validity of a criterion on the i-th line among all normalized validity in the fusion domain; According to the BPA function, the basic probability distribution function value of each criterion is determined, and according to the basic probability distribution function value, the fusion is performed using formula (6) to obtain a fusion result; in, Where: m1(A i ) is the effectiveness of criterion 1 after normalization on line A; m2(B j ) is the validity of criterion 2 after normalization on line B; C is the body of evidence after fusion of lines AB; Φ is the uncertain part.

9. The method for selecting a single-phase grounding fault line in a resonant grounded power grid according to claim 7, characterized in that: The method for determining the credibility corresponding to each criterion includes: Formula (7) is used to determine the credibility of the predetermined number of criteria entering the fusion domain; Where: Ei is the validity of the criterion, min(Ei) is the minimum value of the validity of all the criteria entering the fusion domain.

10. A single-phase grounding fault line selection device for a resonant grounding power grid, characterized in that: include: The voltage and current module is used to detect the zero-sequence current signal of the current line of the target power grid and transmit it to the control module; A control module is used to obtain the most favorable fault conditions for line selection corresponding to each line selection criterion of the target power grid, and zero-sequence current signals when lines of different lengths are faulty; According to the fault condition that is most conducive to line selection, the zero-sequence current signal of the target power grid is decomposed by EEMD at the time of the fault, and the maximum value of each fault component obtained after the decomposition is obtained, which is used as the ideal partition value corresponding to each line selection criterion; A polynomial curve is fitted according to the ideal partition value, and a fault domain division of the line selection criterion is performed according to the polynomial curve and the current line zero-sequence current signal, wherein the fault domain includes a valid domain, a failure domain and a fusion domain; if all the criterion divisions of the line enter the fusion domain, multiple criterion fusion is performed, and the fault domain division is performed again according to the fusion result; if a predetermined number of criterions enter the fusion domain, the credibility corresponding to each criterion is determined, and the line corresponding to the highest credibility value, the line with only one criterion entering the fusion domain, and the line with the criterion entering the valid domain are selected as the fault line; Display screen, used to display the final line selection result of the control module; The switch-in and switch-out module is connected to the control module and is used to detect the switch status of the target power grid; The power module is used to supply power to the control module and the display screen.

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