CN113640615A - Small current ground fault line selection method based on evidence combined transient state information - Google Patents

Small current ground fault line selection method based on evidence combined transient state information Download PDF

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CN113640615A
CN113640615A CN202110560581.4A CN202110560581A CN113640615A CN 113640615 A CN113640615 A CN 113640615A CN 202110560581 A CN202110560581 A CN 202110560581A CN 113640615 A CN113640615 A CN 113640615A
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zero
line
amplitude
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sequence current
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王宝华
王军楠
蒋海峰
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Nanjing University of Science and Technology
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/08—Locating faults in cables, transmission lines, or networks
    • G01R31/081—Locating faults in cables, transmission lines, or networks according to type of conductors
    • G01R31/085—Locating faults in cables, transmission lines, or networks according to type of conductors in power transmission or distribution lines, e.g. overhead
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/08—Locating faults in cables, transmission lines, or networks
    • G01R31/088—Aspects of digital computing
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/50—Testing of electric apparatus, lines, cables or components for short-circuits, continuity, leakage current or incorrect line connections
    • G01R31/52—Testing for short-circuits, leakage current or ground faults

Abstract

The invention discloses a method for selecting a low-current ground fault line based on stable transient state information of evidence combination, which comprises the following steps: collecting fault signals of a line with a single-phase earth fault, wherein the fault signals comprise a zero-sequence current quintic harmonic amplitude value, a phase signal and an amplitude signal of zero-sequence current active power by using a fault collecting device; selecting an original non-stationary fault signal (transient zero-sequence current of a fault line) to carry out VMD modal decomposition, carrying out Hilbert transform on the central frequency of IMF components to obtain instantaneous frequency distribution, obtaining different groups of decomposition frequencies through different modal decomposition numbers K, respectively calculating different frequency errors, and determining the decomposition frequencies as the optimal decomposition numbers if the frequency deviation requirements are met; and then, converting the amplitude and the phase characteristic of the zero-sequence current fifth harmonic in the steady state period and the amplitude of the active power into corresponding fault measure functions, calculating the energy proportion of the first IMF component after VMD decomposition, then performing evidence fusion by using a D-S evidence theory, calculating a fused line function trust value, finally determining a fault line, and finishing line selection. Through testing, the D-S information fusion method can improve the selection accuracy of the line.

Description

Small current ground fault line selection method based on evidence combined transient state information
Technical Field
The invention belongs to the technical field of small current ground fault line selection, and particularly relates to a small current ground fault line selection method based on evidence combined stable transient state information
Background
The medium voltage distribution network with the voltage of 35KV and below in China adopts a neutral point ungrounded mode to improve the reliability of system operation, and is provided with a fault line selection device, so that permanent faults can be eliminated to a certain extent, and the working difficulty of operation and maintenance personnel is reduced. For a power distribution network containing a cable line, because the capacitance current to the ground is large, in order to reduce the capacitance current of a fault point and avoid overvoltage and periodic arcing, a neutral point adopts a mode of grounding through an arc suppression coil, but simultaneously, huge challenges are brought to accurate fault line selection.
At the earliest, the line selection method in the steady state period is mostly adopted for line selection, but most of the line selection methods in the steady state period are suitable for ungrounded systems, the line selection can be accurately performed only when the fault capacitance current value is large enough, the existing grounding mode is mostly that the line is grounded through an arc suppression coil, and the steady state capacitance current compensated by the arc suppression coil is close to zero, so that the line selection method by utilizing steady state signals becomes a little bit less important, and a plurality of researchers turn the attention to the line selection by utilizing signals in the transient state period. However, the transient signal line selection method also has disadvantages, and for the fault conditions of large ground resistance and fault point position far from the bus position, the line selection by using the transient signal is also redundant and insufficient, so some researchers turn the attention to the fusion criterion method at present.
The line selection method based on the steady state information is that after a fault occurs, the line selection is carried out by utilizing the steady state electrical information quantity, and the method has higher practicability in a neutral point ungrounded system, but is not applicable to a system which is grounded through an arc suppression coil. Therefore, a line selection method based on transient information is proposed, namely, line selection is performed by using transient information of one to two periods after a fault occurs, and due to the fact that fault time is short, fault signals are difficult to extract, and noise interference exists, wrong line selection can also occur in the line selection method. Meanwhile, with the development of urban power distribution networks, the automation degree is higher and higher, the accuracy and rapidity of fault line selection are also required to be higher and higher, and in the face of complex and changeable power grid conditions, the accuracy rate is not very high only by a single line selection method, so that some students turn the attention to the research of multi-information fusion line selection.
The invention carries out modeling analysis on the small current grounding system, extracts the stable transient fault information for analysis, and selects a method of comparing the fifth harmonic of zero sequence current with the amplitude of active power ratio and a line selection method based on the energy proportion under VMD for fault information fusion, thereby overcoming the defects of a single line selection method to a certain extent and improving the accuracy of line selection.
Disclosure of Invention
The invention aims to provide a method for selecting a low-current ground fault line based on stable transient state information of evidence combination, which aims to solve the problem that a fault line is difficult to correctly select due to unobvious fault characteristics when a long-distance and high-resistance ground is connected.
The technical solution for realizing the purpose of the invention is as follows: a method for selecting a line of a low-current ground fault based on stable transient state information of evidence combination comprises the following steps:
step 1, acquiring electrical quantity information of a line when a single-phase earth fault occurs by using a fault acquisition device, wherein the electrical quantity information comprises a bus, a zero-sequence current quintuple harmonic signal and a zero-sequence active power signal of each branch feeder line in a steady state period, and a transient zero-sequence current signal in a transient state period;
step 2, VMD decomposition is carried out on the extracted transient zero sequence current signals, frequency deviation is solved, and the optimal decomposition number K is calculated;
step 3, calculating the energy proportion occupied by the first IMF component of the transient zero-sequence current signal after VMD decomposition under the optimal decomposition number K;
step 4, calculating the amplitude and angle characteristics of the quintic harmonic signal of the zero-sequence current and the amplitude characteristics of the zero-sequence active power signal according to the quintic harmonic signal of the zero-sequence current and the zero-sequence active power signal in the steady state period;
and 5, fusing the energy proportion occupied by the first IMF component, the amplitude and angle characteristics of the zero-sequence current fifth harmonic signal and the amplitude characteristics of the zero-sequence active power signal based on the D-S evidence theory to obtain a fault trust value, and completing fault line selection.
Further, step 2, performing VMD decomposition on the extracted transient zero-sequence current, solving the frequency deviation and calculating the optimal decomposition number K, wherein the specific method comprises the following steps:
step 2-1, initializing the decomposition number K to be 2, punishment factor alpha to be 2000, and carrying out VMD decomposition on the signal;
step 2-2, carrying out Hilbert transformation on each IMF component obtained by adopting VMD algorithm decomposition to obtain distribution of instantaneous frequency;
step 2-3, defining the absolute deviation delta of the instantaneous frequency of the IMF componentkCalculating the absolute deviation of the instantaneous frequency of the IMF component as the ratio of the average value of the absolute value of the difference between the instantaneous frequency and the central frequency to the central frequency, wherein the calculation formula is as follows:
Figure BDA0003078804030000021
in the formula, N is the number of time sampling points; f. ofk(t) is the instantaneous frequency of the kth IMF component; ω is the center frequency of the IMF component;
step 2-4, if the absolute deviation deltaKIf the deviation is less than the specified deviation value, adding 1 to the decomposition number K, repeating the steps, and performing VMD decomposition again until the deviation delta isKAnd selecting the previous decomposition number as the optimal decomposition number when the deviation is not less than the specified value.
Further, in step 3, calculating an energy proportion occupied by a first IMF component of the transient zero-sequence current signal after VMD decomposition under the optimal decomposition number K, and the specific method is as follows:
step 3.1, taking a first IMF component of each line transient zero-sequence current signal after VMD decomposition, calculating the frequency band energy of the first IMF component, and marking as E11、E21、…、El1Wherein l is the number of lines on the bus;
step 3.2, respectively obtaining a correlation coefficient of the first IMF component of the transient zero sequence current signal of each line, and recording the correlation coefficient as P11，P21，…，Pl1Setting the total energy E of the transient zero-sequence current signallThen the correlation coefficient Pl1Comprises the following steps:
Figure BDA0003078804030000031
and 3.3, multiplying the correlation coefficient of each feeder line obtained by calculation by the frequency band energy respectively, and recalculating the frequency band energy of the first IMF component of each line, which is recorded as E'11、E′21、...、E′l1；
E′l1＝Pl1gEl1 (3)
Step 3.4, adding the frequency band energy of the first IMF of each line newly calculated in the step 3.3, and performing normalization processing to obtain the energy proportion Q of the first IMF of each line1，Q2，…，Ql。
Further, step 4, according to the fifth harmonic signal of the zero-sequence current and the zero-sequence active power signal in the steady state period, calculating the amplitude and angle characteristics of the fifth harmonic signal of the zero-sequence current and the amplitude characteristics of the zero-sequence active power signal, and the specific method is as follows:
(1) amplitude feature
For a system with l lines, the zero sequence current fifth harmonic signal amplitude and the zero sequence active power signal amplitude of each feeder line are assumed to be respectively (A)1，A2，…，Al)、(B1，B2，…，Bl) The amplitude characteristic of the fifth harmonic signal of the zero sequence current of the ith feeder line and the amplitude characteristic of the zero sequence active power signal are Fi、HiComprises the following steps:
Figure BDA0003078804030000032
preprocessing the original amplitude data according to a formula (4) to obtain amplitude characteristic data with a numerical value interval of [0,1 ];
(2) angular characteristics
Suppose that the phase angle of the zero sequence current quintic harmonic signal of each feeder line is P1，P2，…，PnFirstly, the normalized phase angle Ad corresponding to the ith feeder line is calculatediAnd then calculating the zero sequence current fifth harmonic signal angle characteristic G of the i feeder linesiComprises the following steps:
Figure BDA0003078804030000041
after the original phase angle data are preprocessed according to the formula (5), the angle data with variable values are unified at 0 and 1.
Further, step 5, based on a D-S evidence theory, fusing the energy proportion occupied by the first IMF component, the zero-sequence current fifth harmonic signal amplitude and angle characteristics, and the amplitude characteristics of the zero-sequence active power signal to obtain a fault trust value, and completing fault line selection, specifically including:
step 5.1, setting Fi(x) Is bpa function of the amplitude characteristic of the zero sequence fifth harmonic signal of the ith feeder line, Gi(x) Bpa function of the phase angle characteristic of the zero sequence fifth harmonic signal of the ith feeder line, Hi(x) Is bpa function of the zero sequence active power amplitude of the ith feeder line, Qi(x) And performing evidence combination on the ith feeder line based on bpa functions of the energy proportion occupied by the first IMF component of the VMD algorithm, wherein the bpa functions mi (x) after the evidence combination are as follows:
Figure BDA0003078804030000042
Figure BDA0003078804030000043
wherein the symbols
Figure BDA0003078804030000044
Referred to as quadrature operation, also referred to as quadrature sum;
and 5.2, selecting the line with the maximum combined confidence function, comparing the maximum combined confidence function with a confidence threshold value T of 0.5, and if the maximum combined confidence function is greater than or equal to T, determining that the line is a fault line, otherwise, failing to select the line.
A small current ground fault line selection system based on stable transient state information of evidence combination is disclosed.
A computer device comprising a memory, a processor and a computer program stored on the memory and executable on the processor, the processor implementing a low current ground fault line selection based on evidence combined transient information when executing the computer program based on the method.
A computer-readable storage medium, on which a computer program is stored which, when being executed by a processor, carries out a low-current ground fault line selection based on evidence-combined transient information based on the method.
Compared with the prior art, the invention has the following remarkable advantages: (1) the method can solve the problem of VMD decomposition number, can well select the optimal decomposition number, and does not have the problems of modal aliasing or insufficient modal decomposition display. (2) Through the fusion of the steady-state fault information and the transient-state fault information, the accuracy of fault line selection can be greatly improved.
Drawings
FIG. 1 is a diagram illustrating an instantaneous frequency-based VMD modal decomposition number selection method according to the present invention.
Fig. 2 is a diagram of a low current grounding system with four feed lines in an embodiment of the present invention.
Fig. 3 is a VMD decomposition result when the IMF decomposition number is 7 in the embodiment of the present invention.
FIG. 4 is a graph of the instantaneous frequency for an IMF decomposition number of 7 in accordance with an embodiment of the present invention.
FIG. 5 is a flow chart of line selection based on D-S evidence theory.
Detailed Description
In order to make the objects, technical solutions and advantages of the present application more apparent, the present application is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present application and are not intended to limit the present application.
The invention discloses a small current ground fault line selection method based on evidence combined stable transient state information, which selects modal decomposition numbers by performing HHT conversion on each modal decomposition amount and solving instantaneous frequency calculation frequency deviation of each modal decomposition amount, makes full use of different frequency deviations generated by the modal decomposition numbers to select the decomposition numbers, selects the most suitable modal decomposition numbers, and is convenient to prepare for subsequent fusion characteristic quantities, and comprises the following specific steps:
step 1, acquiring electrical quantity information of a line when a single-phase earth fault occurs by using a fault acquisition device, wherein the electrical quantity information comprises a bus, a zero-sequence current quintuple harmonic signal and a zero-sequence active power signal of each branch feeder line in a steady state period, and a transient zero-sequence current signal in a transient state period;
step 2, VMD decomposition is carried out on the extracted transient zero sequence current, frequency deviation is solved, and the optimal decomposition number K is calculated;
the VMD may decompose the signal into a plurality of eigenmode functions, which may be defined as a frequency modulated amplitude modulated signal, which may be expressed as:
Figure BDA0003078804030000051
in the formula uk(t) represents an amplitude modulated frequency modulated signal; a. thek(t) represents an envelope signal;
Figure BDA0003078804030000052
indicating the instantaneous phase.
Introducing an extended Lagrange function to change the variational problem with constraint conditions into the variational problem without constraint conditions, wherein the expression is as follows:
Figure BDA0003078804030000061
in the formula, alpha represents a secondary penalty factor; λ denotes the Lagrange multiplier.
By constantly updating
Figure BDA0003078804030000062
And (3) solving the optimal solution of the formula (6), and updating the formula as follows:
Figure BDA0003078804030000063
Figure BDA0003078804030000064
Figure BDA0003078804030000065
in the formula (I), the compound is shown in the specification,
Figure BDA0003078804030000066
to represent
Figure BDA0003078804030000067
Wiener filtering of (1);
Figure BDA0003078804030000068
representing a modal power spectrum center of gravity; to pair
Figure BDA0003078804030000069
Performing inverse Fourier transform to obtain the real part of the result as uk(t)。
The given iteration termination condition in the updating process is as follows:
Figure BDA00030788040300000610
inputting an original non-stationary signal, namely a transient zero-sequence current signal, and executing the following specific steps:
step 2-1, initializing the decomposition number K to be 2, punishment factor alpha to be 2000, and carrying out VMD decomposition on the signal;
step 2-2, carrying out Hilbert transformation on each IMF component obtained by adopting VMD algorithm decomposition to obtain distribution of instantaneous frequency;
step 2-3, defining the absolute deviation delta of the instantaneous frequency of the IMF componentkCalculating the absolute deviation of the instantaneous frequency of the IMF component as the ratio of the average value of the absolute value of the difference between the instantaneous frequency and the central frequency to the central frequency, wherein the calculation formula is as follows:
Figure BDA00030788040300000611
in the formula, N is the number of time sampling points; f. ofk(t) is the instantaneous frequency of the kth IMF component; ω is the center frequency of the IMF component;
step 2-4, if the absolute deviation deltaKIf the deviation is less than the specified deviation value, adding 1 to the decomposition number K, repeating the steps, and performing VMD decomposition again until the deviation delta isKAnd selecting the previous decomposition number as the optimal decomposition number when the deviation is not less than the specified value.
The number of decompositions K is generally set to 2 to 9 times, and if and only if the frequency deviation is smaller than a prescribed value, the condition is satisfied, and if the frequency deviation is larger than the prescribed value, the previous number of decompositions needs to be selected as the optimum number of decompositions. According to the regulations related to frequency deviation in the related regulations, the limit value of the frequency deviation of the power system under normal operation conditions is +/-0.2 Hz (49.8-50.2), and when the capacity of the power system is not large, the upper limit and the lower limit of the frequency deviation can be relaxed to be +/-0.5 Hz (49.5-50.5). On this basis, the absolute deviation limit of the instantaneous frequency is set to 2%, and when the absolute deviation exceeds the limit, the instantaneous frequency of the IMF component is seriously deviated from the central frequency.
Step 3, calculating the energy proportion occupied by the first IMF component under the optimal decomposition number K;
when a single-phase earth fault occurs in a system, collecting a bus and transient zero-sequence current signals generated by each feeder line to perform VMD (virtual matrix decomposition), taking a first IMF (inertial measurement function) component of each line transient zero-sequence current signal after VMD decomposition, calculating the frequency band energy of the first IMF component, and recording as E11、E21、…、El1Wherein l is the number of the lines on the bus, and the following steps are executed in the same way:
step 3.1, respectively obtaining a correlation coefficient of a first IMF component of each line transient zero sequence current signal, and recording the correlation coefficient as P11，P21，…，Pl1。
The degree of similarity between two signals to be compared is generally expressed by a correlation coefficient, assuming that the input signal is the band energy of the first IMF component, denoted as E11、E21、…、El1And total energy E of the transient zero-sequence current signallThen the correlation coefficient Pl1Comprises the following steps:
Figure BDA0003078804030000071
and performing VMD decomposition on the transient zero-sequence current signal of each line, and comparing the obtained first IMF component with the original signal to obtain a correlation coefficient between each first modal component IMF and the transient zero-sequence current signal.
And 3.2, multiplying the correlation coefficient by the frequency band energy, and recalculating the frequency band energy of the first IMF component of each line, which is recorded as E'11、E′21、...、E′l1。
E′l1＝Pl1gEl1 (15)
Step 3.3, adding the frequency band energy of the first IMF1 of each line newly calculated in the step 3.2, and performing normalization processing to obtain the energy proportion Q of the first IMF of each line1，Q2，…，Ql。
Step 4, calculating the amplitude and angle characteristics of the quintic harmonic signal of the zero-sequence current and the amplitude characteristics of the zero-sequence active power signal according to the quintic harmonic signal of the zero-sequence current and the zero-sequence active power signal in the steady state period;
(1) amplitude characteristic
For a system with l lines, suppose the zero sequence current quintic harmonic signal amplitude sum of each feeder lineThe amplitude characteristics of the zero sequence active power signal are respectively (A)1，A2，…，Al)、(B1，B2，…，Bl) The amplitude characteristic of the fifth harmonic signal of the zero sequence current of the ith feeder line and the amplitude characteristic of the zero sequence active power signal are Fi、HiComprises the following steps:
Figure BDA0003078804030000081
preprocessing the original amplitude data according to a formula (4) to obtain amplitude characteristic data with a numerical value interval of [0,1 ];
(2) angular characteristics
Suppose that the phase angle of the zero sequence current quintic harmonic signal of each feeder line is P1，P2，…，PnFirstly, the normalized phase angle Ad corresponding to the ith feeder line is calculatediAnd then calculating the zero sequence current fifth harmonic signal angle characteristic G of the i feeder linesiComprises the following steps:
Figure BDA0003078804030000082
after the original phase angle data are preprocessed according to the formula (5), the angle data with variable values are unified at 0 and 1
Step 5, D-S evidence combination is carried out, and the combined function trust value is calculated
Given the specific definition of the bpa function: given the recognition framework Θ, at power set 2 of ΘΘThe function m (g) is defined as above, so that 2Θ→[0,1]And satisfies the following conditions:
m (phi) is 0 and
Figure BDA0003078804030000083
weighing m (g) as 2ΘBasic probability assignment function (basic probability assignment) or basic confidence assignment function, which is abbreviated as bpa function.
Step 5.1, setting Fi(x) As a function of the magnitude of the fifth harmonic bpa, Gi(x) Is fifth harmonicBpa function of wave phase angle, Hi(x) As a function of the magnitude of the active power, bpa, Qi(x) Is a function bpa of the energy contribution occupied by the first IMF component based on the VMD algorithm. The combination is carried out, and the bpa function after evidence combination is as follows:
Figure BDA0003078804030000091
Figure BDA0003078804030000092
wherein the symbols
Figure BDA0003078804030000093
Referred to as quadrature operation, also referred to as quadrature sum.
And 5.2, selecting the line with the maximum combined confidence function, comparing the maximum combined confidence function with a confidence threshold value T of 0.5, and if the maximum combined confidence function is greater than or equal to T, determining that the line is a fault line, otherwise, failing to select the line.
In conclusion, the amplitude and phase characteristics of the active power including the fifth harmonic of the zero-sequence current and the zero-sequence current are given, the original non-stationary fault signal is selected for VMD modal decomposition, obtaining different groups of decomposition frequencies through different decomposition numbers, respectively calculating different frequency errors, determining the decomposition frequencies as the optimal decomposition numbers if the frequency deviation requirements are met, performing Hilbert transform on the calculated central frequency, determining the decomposition numbers as the optimal decomposition numbers, next, converting the amplitude of the fifth harmonic of the steady-state zero-sequence current and the amplitude of the phase and the active power into corresponding fault measure functions, calculating the energy proportion of the first IMF component of the transient zero-sequence current after VMD decomposition, normalizing the energy proportion, and (4) performing evidence fusion by using a D-S evidence theory, calculating a trust value, finally determining a fault line, and completing line selection. The accuracy of line selection is improved to a certain extent, and the accuracy of low-current grounding line selection is improved.
Examples
To verify the validity of the inventive scheme, the following simulation experiment was performed.
And with reference to fig. 2, a 35KV/10KV low-current grounding system model with 4 outgoing lines is built by using SimulinK, wherein the power supply adopts a three-phase power supply, when a grounding mode that a neutral point is not grounded is adopted, the connection mode of the three-phase power supply is a Y-type connection, and the capacity of a transformer in the system is 6 MVA. When the neutral point is grounded through the arc suppression coil, the power supply is a three-phase power supply, and the transformer adopts a Y/Yn wiring mode.
The overhead line has a positive sequence parameter of R1＝0.294Ω/Km，L1＝1.097mH/Km，C10.048 μ F/Km; zero sequence parameter is R0＝0.5691Ω/Km，L0＝4.155mH/Km，C0Line L1 was 15Km in length, line L2 was 20Km in length, line L3 was 25Km in length, and line L4 was 30Km in overall length, 0.055 μ F/Km.
The total capacitance current to the ground of the system with the neutral point grounded through the arc suppression coil is as follows:
Figure BDA0003078804030000094
when the overcompensation degree is 10%, the inductance value of the arc suppression coil is as follows:
Figure BDA0003078804030000101
and (3) building a model, running simulation, and extracting a zero-sequence current fifth harmonic signal, a zero-sequence active power signal and a transient zero-sequence current signal in a transient period in a steady state period according to the steps 1 and 2.
1) VMD decomposition is carried out on the extracted transient zero sequence current, frequency deviation is solved, and the optimal decomposition number K is calculated
TABLE 1 IMF Absolute deviation of instantaneous frequency
Figure BDA0003078804030000102
As shown in table 1, the absolute deviation of the instantaneous frequency gradually increases in the process of increasing the modal decomposition from 2 times to 9 times. When the number of decompositions is 2, the instantaneous frequency deviation of the IMF1 is very small, until the number of modal decompositions is 7, the absolute deviation of the instantaneous frequency of the IMF1 is still very small, when the number of modal decompositions is 8, the absolute error of the instantaneous frequency of the IMF1 reaches 9.63%, and when the number of decompositions is 9, the absolute error of the instantaneous frequency of the IMF is already very large. According to the related regulations on frequency deviation, under normal operation conditions of the power system, the limit value of the frequency deviation is +/-0.2 Hz (49.8-50.2), and when the capacity of the power system is not large, the upper limit and the lower limit of the frequency deviation can be relaxed to +/-0.5 Hz (49.5-50.5). On this basis, the absolute deviation limit of the instantaneous frequency is set to 2%, and when the absolute deviation exceeds the limit, the instantaneous frequency of the IMF component is seriously deviated from the central frequency. According to the frequency deviation of the instantaneous frequency in table 1, the optimal number of decompositions for VMD is 7, i.e., K is 7.
2) Calculating the energy proportion occupied by the first IMF of each line when the optimal decomposition number K is 7
Table 2 IMF1 band energy and correlation coefficient for each line
Figure BDA0003078804030000103
TABLE 3 band energy and specific gravity of the first IMF of each line
Figure BDA0003078804030000104
Figure BDA0003078804030000111
3) Establishing fault measure function based on amplitude characteristic and fault measure function based on angle characteristic
Aiming at the characteristics of the fifth harmonic and the active power amplitude of the zero-sequence current:
TABLE 4 data preprocessing of neutral over arc suppression coil grounding system
Figure BDA0003078804030000112
Phase angle characteristic for the fifth harmonic of zero sequence current:
TABLE 5 data preprocessing of neutral over arc suppression coil grounding system
Figure BDA0003078804030000113
4) Fifth harmonic amplitude and phase angle characteristics Fi、GiActive power amplitude characteristic HiEnergy specific gravity Q of the first IMF component under VMD algorithmiPerforming D-S evidence combination
In the simulation model, the neutral point is grounded through an arc suppression coil, the initial angle is selected to be 0 degrees, the fault line is L4, the A phase grounding fault occurs, the distance from the bus is 25Km, the fault grounding resistance is Rf 2000 omega,
as can be seen from table 6, the maximum confidence value of the merged line L4 is 0.9746 and is greater than the threshold value set in advance by 0.5, so that it can be determined that the line 4 has a fault, and the line selection is successful.
TABLE 6 calculation of confidence values for the example function
Figure BDA0003078804030000114
In order to verify that the line selection scheme has good universality in an arc suppression coil grounding system, various fault conditions are utilized to carry out simulation experiments, and specific results are shown in the following table:
TABLE 7 line selection results via arc suppression coil system ground fusion
Figure BDA0003078804030000121
As shown in table 7 above, the function trust value in the table is the maximum value of the four lines after the 4 criteria and the fusion, and it can be seen that under the condition of a single criterion, the line can be correctly selected mostly, but the trust value is gradually reduced along with the increase of the fault distance, the change of the initial angle and the ground resistance, where when the fault line is set to L4, the fault distance is 25Km, the fault angle is 0 °, and the fault resistance is 2000 Ω, the misjudgment occurs when the judgment is performed by using the zero-sequence current quintic harmonic amplitude method, the fault line is determined to be L3, and the actually set fault line is L4; however, after the information fusion, the correct fault line is selected, and the occurrence of the misjudgment condition is effectively avoided.
The technical features of the above embodiments can be arbitrarily combined, and for the sake of brevity, all possible combinations of the technical features in the above embodiments are not described, but should be considered as the scope of the present specification as long as there is no contradiction between the combinations of the technical features.
The above-mentioned embodiments only express several embodiments of the present application, and the description thereof is more specific and detailed, but not construed as limiting the scope of the invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the concept of the present application, which falls within the scope of protection of the present application. Therefore, the protection scope of the present patent shall be subject to the appended claims.

Claims (8)

1. A method for selecting a line of a low-current ground fault based on stable transient state information of evidence combination is characterized by comprising the following steps:
step 1, acquiring electrical quantity information of a line when a single-phase earth fault occurs by using a fault acquisition device, wherein the electrical quantity information comprises a bus, a zero-sequence current quintuple harmonic signal and a zero-sequence active power signal of each branch feeder line in a steady state period, and a transient zero-sequence current signal in a transient state period;
step 2, VMD decomposition is carried out on the extracted transient zero sequence current signals, frequency deviation is solved, and the optimal decomposition number K is calculated;
step 3, calculating the energy proportion occupied by the first IMF component of the transient zero-sequence current signal after VMD decomposition under the optimal decomposition number K;
step 4, calculating the amplitude and angle characteristics of the quintic harmonic signal of the zero-sequence current and the amplitude characteristics of the zero-sequence active power signal according to the quintic harmonic signal of the zero-sequence current and the zero-sequence active power signal in the steady state period;
and 5, fusing the energy proportion occupied by the first IMF component, the amplitude and angle characteristics of the zero-sequence current fifth harmonic signal and the amplitude characteristics of the zero-sequence active power signal based on the D-S evidence theory to obtain a fault trust value, and completing fault line selection.
2. The method for small-current ground fault line selection based on evidence-combined transient state information according to claim 1, wherein step 2, VMD decomposition is performed on the extracted transient state zero sequence current, frequency deviation is solved, and the optimal decomposition number K is calculated, and the method specifically comprises the following steps:
step 2-1, initializing the decomposition number K to be 2, punishment factor alpha to be 2000, and carrying out VMD decomposition on the signal;
step 2-2, carrying out Hilbert transformation on each IMF component obtained by adopting VMD algorithm decomposition to obtain distribution of instantaneous frequency;
step 2-3, defining the absolute deviation delta of the instantaneous frequency of the IMF componentkCalculating the absolute deviation of the instantaneous frequency of the IMF component as the ratio of the average value of the absolute value of the difference between the instantaneous frequency and the central frequency to the central frequency, wherein the calculation formula is as follows:
Figure FDA0003078804020000011
in the formula, N is the number of time sampling points; f. ofk(t) is the instantaneous frequency of the kth IMF component; ω is the center frequency of the IMF component;
step 2-4, if the absolute deviation deltaKIf the deviation is less than the specified deviation value, adding 1 to the decomposition number K, repeating the steps, and performing VMD decomposition again until the deviation delta isKAnd selecting the previous decomposition number as the optimal decomposition number when the deviation is not less than the specified value.
3. The method for small-current ground fault line selection based on evidence-combined transient state information according to claim 1, wherein in step 3, the energy proportion of the first IMF component of the transient zero-sequence current signal after VMD decomposition under the optimal decomposition number K is calculated, and the method specifically comprises:
step 3.1, taking a first IMF component of each line transient zero-sequence current signal after VMD decomposition, calculating the frequency band energy of the first IMF component, and marking as E11、E21、…、El1Wherein l is the number of lines on the bus;
step 3.2, respectively obtaining a correlation coefficient of the first IMF component of the transient zero sequence current signal of each line, and recording the correlation coefficient as P11，P21，…，Pl1Setting the total energy E of the transient zero-sequence current signallThen the correlation coefficient Pl1Comprises the following steps:
Figure FDA0003078804020000021
and 3.3, multiplying the correlation coefficient of each feeder line obtained by calculation by the frequency band energy respectively, and recalculating the frequency band energy of the first IMF component of each line, which is recorded as E'11、E′21、...、E′l1；
E′l1＝Pl1gEl1 (3)
Step 3.4, adding the frequency band energy of the first IMF of each line newly calculated in the step 3.3, and performing normalization processing to obtain the energy proportion Q of the first IMF of each line1，Q2，…，Ql。
4. The method for selecting the small-current ground fault line based on the stable transient state information of evidence combination according to claim 1, wherein step 4 is to calculate the amplitude and angle characteristics of the fifth harmonic signal of zero-sequence current and the amplitude characteristics of the zero-sequence active power signal according to the fifth harmonic signal of zero-sequence current and the zero-sequence active power signal in the steady state period, and the specific method is as follows:
(1) amplitude feature
For a system with l lines, the zero sequence current fifth harmonic signal amplitude and the zero sequence active power signal amplitude of each feeder line are assumed to be respectively (A)1，A2，…，Al)、(B1，B2，…，Bl) The amplitude characteristic of the fifth harmonic signal of the zero sequence current of the ith feeder line and the amplitude characteristic of the zero sequence active power signal are Fi、HiComprises the following steps:
Figure FDA0003078804020000022
preprocessing the original amplitude data according to a formula (4) to obtain amplitude characteristic data with a numerical value interval of [0,1 ];
(2) angular characteristics
Suppose that the phase angle of the zero sequence current quintic harmonic signal of each feeder line is P1，P2，…，PnFirstly, the normalized phase angle Ad corresponding to the ith feeder line is calculatediAnd then calculating the zero sequence current fifth harmonic signal angle characteristic G of the i feeder linesiComprises the following steps:
Figure FDA0003078804020000031
after the original phase angle data are preprocessed according to the formula (5), the angle data with variable values are unified at 0 and 1.
5. The method for selecting the line of the small-current ground fault based on the stable transient state information of the evidence combination according to claim 1, wherein in the step 5, based on a D-S evidence theory, the energy proportion occupied by the first IMF component, the amplitude and angle characteristics of the fifth harmonic signal of the zero-sequence current and the amplitude characteristics of the zero-sequence active power signal are fused to obtain a fault trust value, and the line selection of the fault line is completed, and the method comprises the following specific steps:
step 5.1, setting Fi(x) Is bpa function of the amplitude characteristic of the zero sequence fifth harmonic signal of the ith feeder line, Gi(x) Bpa function of the phase angle characteristic of the zero sequence fifth harmonic signal of the ith feeder line, Hi(x) Is bpa function of the zero sequence active power amplitude of the ith feeder line, Qi(x) Performing evidence combination on the ith feeder line based on bpa functions of energy proportion occupied by the first IMF component of the VMD algorithm, wherein the bpa functions m after the evidence combinationi(x) Comprises the following steps:
Figure FDA0003078804020000032
Figure FDA0003078804020000033
wherein the symbols
Figure FDA0003078804020000034
Referred to as quadrature operation, also referred to as quadrature sum;
and 5.2, selecting the line with the maximum combined confidence function, comparing the maximum combined confidence function with a confidence threshold value T of 0.5, and if the maximum combined confidence function is greater than or equal to T, determining that the line is a fault line, otherwise, failing to select the line.
6. A low-current ground fault line selection system based on evidence-combined transient state information, characterized in that the low-current ground fault line selection based on the evidence-combined transient state information is realized based on the method of any one of claims 1 to 5.
7. A computer device comprising a memory, a processor and a computer program stored on the memory and executable on the processor, the processor implementing a low current ground fault line selection based on evidence combined transient state information based on the method of any one of claims 1-5 when the computer program is executed.
8. A computer-readable storage medium, on which a computer program is stored which, when being executed by a processor, carries out a low-current ground-fault line selection based on evidence-combined transient information based on the method of any one of claims 1-5.
CN202110560581.4A 2021-05-21 2021-05-21 Small current ground fault line selection method based on evidence combined transient state information Pending CN113640615A (en)

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