Tree line fault impedance measuring method

Abstract

The invention discloses a tree line fault impedance measurement method, which relates to the technical field of high-voltage tests, and comprises the following steps: measuring leakage current flowing through a whole live tree through a lead suspended at the front section of a fault point, and acquiring voltage signals at two ends of a line to obtain fault point voltage; carrying out noise reduction on the collected leakage current and fault point voltage through wavelet transform; performing quadratic polynomial fitting interpolation on relatively short signal data to align the two signals; the method is used for fault mechanism analysis and fault identification, and has important significance for ensuring the safety of the distribution line and preventing and treating the forest fire caused by the tree line fault.

Description

Technical Field

[0001] The invention relates to the technical field of high voltage testing, in particular to a method for measuring tree-line fault impedance. Background Art

[0002] A tree-line fault in a single-phase high-resistance grounding fault in a power system line refers to a tree contacting a conductor, which in turn causes the conductor to be single-phase grounded. When analyzing the tree-line fault mechanism and fault identification, it is necessary to study how the impedance of the tree changes as the fault develops. The calculation is performed by measuring the leakage current flowing through the tree and the line phase voltage at the fault point. However, there are several problems. First, the leakage current flowing through the tree is difficult to measure; second, the leakage current needs to be measured at the fault point, and measuring the voltage at the fault point requires a power voltage transformer, but this device is not necessarily next to the fault point. Usually, the voltage value of the line can be measured at the beginning and end of the line, but the voltage at the fault point cannot be directly measured. pressure; third, there may be other feeders in the line. If two fault recorders are needed to collect the current waveform signal of the fault point and the voltage waveform signal at both ends of the line respectively, the sampling rates of the signals collected by the two different devices may be different, and even the lengths of the fault signals collected by the two devices may be different due to the difference in equipment; fourth, the current signal collected in the tree-line fault is not only affected by environmental noise, but also by the arc current caused by the gap between the conductor and the tree; the arc resistance is between 1-100Ω, and the tree impedance is between 1-1000kΩ. The effect of arc resistance on the entire transition resistance can be ignored, so the influence of arc current should be excluded.

[0003] Line faults belong to high-resistance faults in high voltage and insulation technology. Tree-line faults are one of the important issues facing power grid security, which can cause transmission line tripping and wildfires. Studying the mechanism of tree-line faults and prevention and control measures is of great significance to preventing wildfires and power grid accidents. At present, most scholars identify faults by analyzing and finding characteristic quantities of electrical quantities such as three-phase voltage, three-phase current, zero-sequence voltage, and zero-sequence current, or by establishing arc models. Few scholars study the mechanism of tree-line faults and fault identification from the perspective of tree equivalent impedance. Studying the characteristics of tree impedance changes is also of great significance for protecting lines and preventing wildfires. In the real experiment, wind caused the separation of trees and lines to generate arc current, which would affect the calculation of equivalent impedance. Summary of the invention

[0004] In order to solve the problems existing in the prior art, the purpose of the present invention is to provide a tree-line fault impedance measurement method, which is used for fault mechanism analysis and fault identification, and is of great significance for ensuring the safety of distribution lines and preventing and controlling forest fires caused by tree-line faults.

[0005] To achieve the above object, the technical solution adopted by the present invention is: a tree-line fault impedance measurement method, comprising the following steps:

[0006] Step 1: Measure the leakage current flowing through the entire live tree by suspending the wire in front of the fault point, and collect voltage signals at both ends of the line to obtain the fault point voltage;

[0007] Step 2: De-noise the collected leakage current and fault point voltage by wavelet transform;

[0008] Step 3, interpolating the shorter signal data by quadratic polynomial fitting to align the two signals;

[0009] Step 4: Calculate the change in equivalent impedance of trees.

[0010] As a further improvement of the present invention, the step 1 specifically comprises the following steps:

[0011] Step 1.1: Increase the 380V voltage of the power distribution cabinet to the required voltage level through a three-phase transformer;

[0012] Step 1.2, one phase of the high voltage three-phase line is in contact with the whole living tree;

[0013] Step 1.3, the end of the high-voltage three-phase line is not loaded, the end of the three-phase conductor is suspended, and a through-core current transformer is installed in front of the fault point. The fault recorder S2 collects the current waveform flowing through the fault phase conductor through the through-core current transformer as the leakage current waveform I0; the fault recorder S1 collects the fault phase voltage waveform U0 through the through-core current transformer.

[0014] As a further improvement of the present invention, the step 2 specifically comprises the following steps:

[0015] Step 2.1, estimate the noise standard deviation σ of the leakage current and the fault point voltage;

[0016] Step 2.2, calculate the soft threshold T, and use the noise standard deviation multiplied by an empirical constant C to determine the threshold: T = σ × C;

[0017] Step 2.3, using wavelet basis to perform multi-layer decomposition on the fault waveform in the current waveform and voltage waveform and extract coefficients;

[0018] Step 2.4, use the soft threshold T calculated based on the background noise to perform denoising and construct the denoised fault waveform;

[0019] Step 2.5: For the entire fault waveform, take the preset time as a period, calculate the root mean square value RMS within the period, and obtain the effective values ​​I1 and U1 of the current signal and the voltage signal.

[0020] As a further improvement of the present invention, the step 2.1 is specifically as follows:

[0021] Calculate the average value μ of the noise waveform of the leakage current and the fault point voltage: In the formula, μ represents the average value of the noise waveform, N represents the total number of data points of the noise waveform signal, and x i Represents each data point of the noise waveform;

[0022] Compute the square of the difference between each data point and the mean and take the average: In the formula, σ 2 represents the square of the noise standard deviation;

[0023] Calculate the noise standard deviation σ:

[0024] As a further improvement of the present invention, in step 2.5, the effective values ​​I1 and U1 of the current signal and the voltage signal are as follows:

[0025] As a further improvement of the present invention, it also includes: using the moving average of the smooth function in matlab to smooth the effective value I1 of the current signal, removing the sudden change current caused by the arc current, and obtaining the effective value I2 of the current signal after smoothing.

[0026] As a further improvement of the present invention, the step 3 specifically comprises the following steps:

[0027] Step 3.1, compare the length of the effective value signal array and calculate the number of points that need to be inserted;

[0028] Step 3.2, evenly distribute the points to be inserted into the shorter signal array;

[0029] Step 3.3: Use polynomial fitting to interpolate: Take the data in multiple data windows before and after the interpolation point and perform polynomial fitting: y = a n x n +a n-1 x n-1 +…+a1x+1; solve the polynomial coefficients a0, a1, …, a by the least squares method n ; Generate a fitting curve, use the fitted polynomial to generate the curve, and calculate the value of the missing point in the middle; insert the missing data points in the fitting curve and align the data to obtain signals U2 and I3.

[0030] As a further improvement of the present invention, the step 4 specifically comprises the following steps:

[0031] Step 4.1, calculate the voltage of the fault phase at the fault point:

[0032] Calculate the line resistance: In the formula, R Lrepresents the resistance of the line, ρ represents the resistivity of the line material, L represents the length of the line, and S is the cross-sectional area of ​​the conductor; calculate the voltage at the fault point: U3 = U2-I3R L ; In the formula, U3 represents the effective value of the fault phase voltage at the fault point, U2 represents the effective value of the voltage signal after alignment, and I3 represents the effective value of the current signal after alignment, that is, the effective value of the leakage current;

[0033] Step 4.2, calculate the tree equivalent impedance change R:

[0034] The beneficial effects of the present invention are:

[0035] The present invention can be used to collect and process electrical signals in tree-line fault experiments, realizes the collection of leakage current of the entire living tree, reduces the influence of arc current on calculated impedance, solves the pain point that dual-fault recorders are difficult to calculate impedance, and improves the accuracy of tree equivalent impedance by calculating the phase voltage at the fault point; the present invention is used for fault mechanism analysis and fault identification, and is of great significance to ensuring the safety of distribution lines and preventing and controlling forest fires caused by tree-line faults. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 is a flow chart of an embodiment of the present invention;

[0037] Figure 2 This is an example diagram of a method for measuring leakage current flowing through a whole living tree in an embodiment of the present invention;

[0038] Figure 3 This is a complete waveform diagram of the current signal in an embodiment of the present invention;

[0039] Figure 4 The waveform diagram of the current signal in the embodiment of the present invention is a waveform diagram in the fault stage after removing the waveforms of the previous and next non-fault stages;

[0040] Figure 5 This is a waveform diagram after threshold denoising obtained through background noise calculation in an embodiment of the present invention;

[0041] Figure 6 is a curve diagram of the effective value change of the current signal after calculation in an embodiment of the present invention;

[0042] Figure 7 4 is a curve diagram of the effective value change of the current signal after smoothing in an embodiment of the present invention. DETAILED DESCRIPTION

[0043] The embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0044] Example

[0045] like Figure 1As shown, a tree-line fault impedance measurement method includes measuring the leakage current flowing through the entire live tree, current signal noise reduction, multiple signal alignment, and calculating the tree equivalent impedance; the following is a specific description:

[0046] 1. A method for measuring leakage current flowing through a whole living tree comprises the following steps:

[0047] Step 1: Use a three-phase transformer to increase the 380V voltage of the distribution incoming cabinet to the required voltage level. Take 10kV as an example. It is equipped with other supporting facilities, such as transformers, circuit breakers, mutual inductors, fault recorders S1 and S2. Figure 2 shown.

[0048] Step 2: One phase of the high voltage three-phase line is in contact with the entire living tree.

[0049] Step 3: The end of the high-voltage three-phase line is not loaded, the end of the three-phase conductor is suspended, and a through-core current transformer is installed in front of the fault point. The fault recorder S2 collects the current waveform flowing through the fault phase conductor through the through-core current transformer as the leakage current waveform I0, as shown in Figure 3 As shown in FIG. 1 , the fault recorder S1 collects the fault phase voltage waveform U0.

[0050] 2. The signal noise reduction method takes the current signal as an example, and includes the following steps:

[0051] Step 1: Estimate the noise standard deviation. The fault phase current waveform I0 collected by the fault recorder S2 includes the noise waveform collected before the fault occurs and the fault current waveform collected after the fault occurs. The standard deviation of the noise is calculated for the noise waveform part.

[0052] Step 1.1: Calculate the mean value μ of the noise waveform:

[0053]

[0054] In the formula, μ represents the average value of the noise waveform, N represents the total number of data points of the noise waveform signal, and x i Represents each data point of the noise waveform.

[0055] Step 1.2: Calculate the square of the difference between each data point and the mean and average them:

[0056]

[0057] Where: 2 Represents the square of the standard deviation.

[0058] Step 1.3: Calculate the standard deviation:

[0059]

[0060] Where: σ represents the standard deviation.

[0061] Step 2: Calculate the soft threshold T and use the noise standard deviation multiplied by an empirical constant C to determine the threshold.

[0062] T=σ×C

[0063] Step 3: Use wavelet base DB2 to perform 3-layer decomposition on the fault waveform in the current waveform and extract coefficients.

[0064] Step 4: Use the soft threshold T calculated based on the background noise to perform denoising and construct the denoised fault waveform.

[0065] Step 5: For the entire fault waveform, take 0.02 seconds as a period, and calculate the root mean square value RMS within the period, which is the effective value I1. The effective value of the voltage signal U1 is processed in the same way.

[0066]

[0067] Where N is the total number of data points every 0.02s in the fault waveform, x i Represents every data point every 0.02s in the noise waveform.

[0068] Step 6: At this time, the processed current waveform still has the sudden current caused by the arc current, but it does not affect the overall trend of the data points. By smoothing the data, the sudden current value can be removed.

[0069] Step 6.1: Use the Moving Average of the smoothing function in Matlab, and the sliding window is set to 100 based on experience.

[0070] Step 6.2: After smoothing, the influence of arc current is removed to obtain the effective value I2 of the current signal.

[0071] 3. A method for aligning multiple signals, comprising the following steps:

[0072] Step 1: Compare the lengths of the two valid value signal arrays and calculate the number of points that need to be inserted.

[0073] Step 2: Evenly distribute the points to be inserted into the shorter signal array.

[0074] Step 3: Interpolate using polynomial fitting.

[0075] Step 3.1: Take the data in the 200 data windows before and after the interpolation point for polynomial fitting, where the data points are (x1, y1), (x2, y2), and so on. Fit according to the formula. This data window can be adjusted as needed:

[0076] y=an x n +a n-1 x n-1 +…+a1x+1

[0077] Step 3.2: Solve the polynomial coefficients a0, a1, …, a by the least squares method n .

[0078] Step 3.3: Generate a fitting curve, use the fitted polynomial to generate a curve, and calculate the values ​​of the missing points in the middle.

[0079] Step 3.4: Insert the missing data points in the fitting curve and align the data to obtain signals U2 and I3. In this process, the current signal is processed as follows: Figure 4-Figure 7 shown.

[0080] 4. The method for calculating the equivalent impedance of trees includes the following steps:

[0081] Step 1: The voltage of the fault phase at the fault point needs to be calculated first.

[0082] Step 1.1: The distribution network line is short, and the voltage drop is mainly caused by the line resistance itself. Calculate the line resistance:

[0083]

[0084] In the formula, R L represents the resistance of the line, ρ represents the resistivity of the line material, L represents the length of the line, and S is the cross-sectional area of ​​the conductor.

[0085] Step: 1.2: Calculate the voltage at the fault point:

[0086] U3=U2-I3R L

[0087] Wherein, U3 represents the effective value of the fault phase voltage at the fault point, U2 represents the effective value of the voltage signal after alignment, and I3 represents the effective value of the current signal after alignment, that is, the effective value of the leakage current.

[0088] Step 2: Calculate the tree equivalent impedance change R by formula:

[0089]

[0090] The above-mentioned embodiments only express the specific implementation of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A method for measuring tree-line fault impedance, characterized in that: The following steps are involved: Step 1: Measure the leakage current flowing through the entire live tree by suspending the wire in front of the fault point, and collect voltage signals at both ends of the line to obtain the fault point voltage; Step 2: De-noise the collected leakage current and fault point voltage by wavelet transform; Step 3, interpolating the shorter signal data by quadratic polynomial fitting to align the two signals; Step 4: Calculate the change in equivalent impedance of trees.

2. The tree-line fault impedance measurement method according to claim 1, characterized in that: The step 1 specifically comprises the following steps: Step 1.1: Increase the 380V voltage of the power distribution cabinet to the required voltage level through a three-phase transformer; Step 1.2, one phase of the high voltage three-phase line is in contact with the whole living tree; Step 1.3, the end of the high-voltage three-phase line is not loaded, the end of the three-phase conductor is suspended, and a through-core current transformer is installed in front of the fault point. The fault recorder S2 collects the current waveform flowing through the fault phase conductor through the through-core current transformer as the leakage current waveform I0; the fault recorder S1 collects the fault phase voltage waveform U0 through the through-core current transformer.

3. The tree-line fault impedance measurement method according to claim 2, characterized in that: The step 2 specifically includes the following steps: Step 2.1, estimate the noise standard deviation σ of the leakage current and the fault point voltage; Step 2.2, calculate the soft threshold T, and use the noise standard deviation multiplied by an empirical constant C to determine the threshold: T = σ × C; Step 2.3, using wavelet basis to perform multi-layer decomposition on the fault waveform in the current waveform and voltage waveform and extract coefficients; Step 2.4, use the soft threshold T calculated based on the background noise to perform denoising and construct the denoised fault waveform; Step 2.5: For the entire fault waveform, take the preset time as a period, calculate the root mean square value RMS within the period, and obtain the effective values ​​I1 and U1 of the current signal and the voltage signal.

4. The tree-line fault impedance measurement method according to claim 3, characterized in that: The step 2.1 is as follows: Calculate the average value μ of the noise waveform of the leakage current and the fault point voltage: In the formula, μ represents the average value of the noise waveform, N represents the total number of data points of the noise waveform signal, and x i Represents each data point of the noise waveform; Compute the square of the difference between each data point and the mean and take the average: In the formula, σ 2 represents the square of the noise standard deviation; Calculate the noise standard deviation σ:

5. The tree-line fault impedance measurement method according to claim 4, characterized in that: In step 2.5, the effective values ​​of the current signal and voltage signal I1 and U1 are as follows:

6. The tree-line fault impedance measurement method according to claim 5, characterized in that: Also includes: In MATLAB, the moving average of the smooth function is used to smooth the effective value I1 of the current signal to remove the sudden change caused by the arc current. After smoothing, the effective value I2 of the current signal is obtained.

7. The tree-line fault impedance measurement method according to claim 6, characterized in that: The step 3 specifically comprises the following steps: Step 3.1, compare the length of the effective value signal array and calculate the number of points that need to be inserted; Step 3.2, evenly distribute the points to be inserted into the shorter signal array; Step 3.3: Use polynomial fitting to interpolate: Take the data in multiple data windows before and after the interpolation point and perform polynomial fitting: y = a n x n +a n-1 x n-1 +…+a1x+1; solve the polynomial coefficients a0, a1, …, a by the least squares method n ; Generate a fitting curve, use the fitted polynomial to generate the curve, and calculate the value of the missing point in the middle; insert the missing data points in the fitting curve and align the data to obtain signals U2 and I3.

8. The tree-line fault impedance measurement method according to claim 7, characterized in that: The step 4 specifically comprises the following steps: Step 4.1, calculate the voltage of the fault phase at the fault point: Calculate the line resistance: In the formula, R L represents the resistance of the line, ρ represents the resistivity of the line material, L represents the length of the line, and S is the cross-sectional area of ​​the conductor; calculate the voltage at the fault point: U3 = U2-I3R L ; In the formula, U3 represents the effective value of the fault phase voltage at the fault point, U2 represents the effective value of the voltage signal after alignment, and I3 represents the effective value of the current signal after alignment, that is, the effective value of the leakage current; Step 4.2, calculate the tree equivalent impedance change R:

Images