A bushing capacitance core damp evaluation method based on high frequency resonance excitation

Abstract

The bushing is an important component for connecting the transformer winding and the conductor, plays an important role in the normal operation of the transformer, and its insulation performance determines the normal operation of the power system. The application discloses a bushing capacitor core moisture evaluation method based on high-frequency resonance excitation. First, a bushing capacitor core test platform based on high-frequency resonance excitation is built. Then, the bushing signal under high-frequency resonance excitation is acquired. Then, the characteristic parameters of the bushing signal under high-frequency resonance excitation are extracted. Finally, the moisture degree of the bushing capacitor core is evaluated based on the characteristic parameters. The method can effectively evaluate the moisture degree of the bushing capacitor core.

Description

Technical Field

[0001] This invention belongs to the field of performance evaluation of transformer bushings, specifically relating to a method for evaluating the moisture content of bushing capacitor cores based on high-frequency resonant excitation. Background Technology

[0002] With the rapid development of modern power systems, bushings, as crucial components connecting transformer windings and conductors, play a vital role in the normal operation of transformers. The insulation performance of bushings determines whether transformers can operate safely and stably. Moisture in the bushing capacitor core is a major factor causing a decline in insulation performance. Moisture can endanger the main insulation of equipment and, in severe cases, cause bushing breakdown and rupture. Common methods for diagnosing bushing moisture include oil chromatography, dielectric loss and capacitance measurement, and frequency domain dielectric spectroscopy. However, oil chromatography is mostly used to detect trace amounts of water in insulating oil and cannot reflect the degree of moisture in the bushing capacitor core, and it can only be used for offline detection. The dielectric loss and capacitance change method is simple to operate, but the detection parameters are affected by many factors, resulting in inaccurate results. Frequency domain dielectric spectroscopy is not sensitive to the detection of slight moisture in bushings. Therefore, a method that can assess the degree of moisture in bushing capacitor cores online is needed. Summary of the Invention

[0003] To overcome the shortcomings of the aforementioned background technology, this invention provides a method for assessing the moisture content of bushing capacitor cores based on high-frequency resonant excitation. This method can effectively and accurately assess the moisture content of bushing capacitor cores, and its steps are as follows:

[0004] Step 1: Building a high-frequency resonant excitation bushing capacitor core test platform

[0005] A test platform for a bushing capacitor core under high-frequency resonant excitation is constructed, which mainly includes a high-voltage DC power supply (1), a protection resistor (2), a high-frequency switch (3), an adjustable inductor (4), a control switch 1 (5), a control switch 2 (6), and a bushing (7). The bushing (7) includes a capacitor core (8), a screen end (9), and a test terminal (10). The two ends of the bushing (7) are connected to a capacitor tester (11), and the screen end (9) and the test terminal (10) are connected to a signal tester (12). The high-voltage DC power supply (1), the protection resistor (2), and the adjustable inductor (4) are connected in series to the control switch 1 (5) and the control switch 2 (6). The high-frequency switch (3) is led out from the middle of the protection resistor (2) and the adjustable inductor (4). The high-voltage DC power supply (1) and the high-frequency switch (3) are both grounded.

[0006] Step 2: Obtain the bushing signal under high-frequency resonant excitation

[0007] (1) The capacitance C of the bushing is obtained by the capacitance tester (11). The inductance at the resonant frequency of 1000K is calculated according to the formula (1) and recorded as L. The adjustable inductor (4) is adjusted to L.

[0008]

[0009] (2) Test the bushing signal under high frequency resonance excitation. At the start of the test, the high frequency switch (3) is in the open state, the control switch 1 (5) is in the closed state, the control switch 2 (6) is in the open state, the high voltage DC power supply (1) and the high frequency switch (3) are both grounded, the high voltage DC power supply (1) inputs DC voltage to the bushing (7) through the protection resistor (2) and the adjustable inductor (4) and the control switch 1 (5), close the high frequency switch (3), and the signal tester (12) reads the high frequency resonance signal s1 of the bushing end screen from the end screen end (9); open the high frequency switch (3), the control switch 1 (5) is in the open state, the control switch 2 (6) is in the closed state, the high voltage DC power supply (1) inputs DC voltage to the bushing (7) through the protection resistor (2) and the adjustable inductor (4) and the control switch 2 (6), close the high frequency switch (3), and the signal tester (12) reads the high frequency resonance signal s2 of the bushing test end from the test terminal (10);

[0010] Step 3: Extracting the characteristic parameters of the bushing signal under high-frequency resonant excitation

[0011] The oscillation wave signal s1 read from the end screen of the bushing and the oscillation wave signal s2 read from the test end of the bushing are converted into a time-frequency graph by the STFT short-time Fourier transform. The window function of the short-time Fourier transform is the Hamming window. Then the time-frequency graph is grayscaled to form a gray-level co-occurrence matrix x(i,j) with a size of n×n.

[0012] The gray-level co-occurrence matrix is ​​reduced in dimension to obtain matrix y(i,j), calculated as follows:

[0013]

[0014] i and j are the coordinates of the co-occurrence matrix, λ is the dimensionality reduction factor related to the dimension of the co-occurrence matrix, with a value of 2, max(h(i,j)) is the maximum value in the gray-level co-occurrence matrix, and min(h(i,j)) is the minimum value in the gray-level co-occurrence matrix;

[0015] The expression for the dimension-reduced gray-level co-occurrence matrix y(i,j) is as follows:

[0016]

[0017] Calculate the contrast (con) of the gray-level co-occurrence matrix:

[0018]

[0019] Where y T (i,j) is the transpose of the gray-level co-occurrence matrix, expressed as follows:

[0020]

[0021] The characteristic parameters of the transformer bushing oscillation signal are obtained, and the expression for the characteristic parameters of the oscillation signal s1 is as follows:

[0022]

[0023] The characteristic parameter expressions for the oscillating wave signal s2 are as follows:

[0024]

[0025] Step 4: Assess the moisture level of the capacitor core in the bushing.

[0026] Calculate the moisture factor of the bushing capacitor core:

[0027]

[0028] The formula for calculating the evaluation coefficient ε of the moisture content of the bushing capacitor core is as follows:

[0029]

[0030] Where η e η is the moisture factor when the casing has no moisture defects. b The moisture factor when the casing has moisture defects;

[0031] When the evaluation coefficient is 0 < ε < 4.2, it indicates that the bushing capacitor core has no obvious moisture defects;

[0032] When the evaluation coefficient ε≥4.2, it indicates that the core of the bushing capacitor is severely damp and needs to be repaired or replaced. Attached Figure Description

[0033] Figure 1 Flowchart of a method for assessing moisture in bushing capacitor cores based on high-frequency resonant excitation

[0034] Figure 2 Schematic diagram of the test platform for high-frequency resonant excitation of the bushing capacitor core. Detailed Implementation

[0035] The following is a more detailed explanation with reference to the accompanying diagram. The specific methods and steps are as follows:

[0036] Step 1: Building a high-frequency resonant excitation bushing capacitor core test platform

[0037] A test platform for a bushing capacitor core under high-frequency resonant excitation is constructed, which mainly includes a high-voltage DC power supply (1), a protection resistor (2), a high-frequency switch (3), an adjustable inductor (4), a control switch 1 (5), a control switch 2 (6), and a bushing (7). The bushing (7) includes a capacitor core (8), a screen end (9), and a test terminal (10). The two ends of the bushing (7) are connected to a capacitor tester (11), and the screen end (9) and the test terminal (10) are connected to a signal tester (12). The high-voltage DC power supply (1), the protection resistor (2), and the adjustable inductor (4) are connected in series to the control switch 1 (5) and the control switch 2 (6). The high-frequency switch (3) is led out from the middle of the protection resistor (2) and the adjustable inductor (4). The high-voltage DC power supply (1) and the high-frequency switch (3) are both grounded.

[0038] Step 2: Obtain the bushing signal under high-frequency resonant excitation

[0039] (1) The capacitance C of the bushing is obtained by the capacitance tester (11). The inductance at the resonant frequency of 1000K is calculated according to the formula (1) and recorded as L. The adjustable inductor (4) is adjusted to L.

[0040]

[0041] (3) Test the bushing signal under high frequency resonance excitation. At the start of the test, the high frequency switch (3) is in the open state, the control switch 1 (5) is in the closed state, the control switch 2 (6) is in the open state, the high voltage DC power supply (1) and the high frequency switch (3) are both grounded, the high voltage DC power supply (1) inputs DC voltage to the bushing (7) through the protection resistor (2) and the adjustable inductor (4) and the control switch 1 (5), close the high frequency switch (3), and the signal tester (12) reads the high frequency resonance signal s1 of the bushing end screen from the end screen end (9); open the high frequency switch (3), the control switch 1 (5) is in the open state, the control switch 2 (6) is in the closed state, the high voltage DC power supply (1) inputs DC voltage to the bushing (7) through the protection resistor (2) and the adjustable inductor (4) and the control switch 2 (6), close the high frequency switch (3), and the signal tester (12) reads the high frequency resonance signal s2 of the bushing test end from the test terminal (10);

[0042] Step 3: Extracting the characteristic parameters of the bushing signal under high-frequency resonant excitation

[0043] The oscillation wave signal s1 read from the end screen of the bushing and the oscillation wave signal s2 read from the test end of the bushing are converted into a time-frequency graph by the STFT short-time Fourier transform. The window function of the short-time Fourier transform is the Hamming window. Then the time-frequency graph is grayscaled to form a gray-level co-occurrence matrix x(i,j) with a size of n×n.

[0044] The gray-level co-occurrence matrix is ​​reduced in dimension to obtain matrix y(i,j), calculated as follows:

[0045]

[0046] i,j are the coordinates of the co-occurrence matrix, λ is the dimensionality reduction factor related to the dimension of the co-occurrence matrix, with a value of 2, max(h(i,j)) is the maximum value in the gray-level co-occurrence matrix, and min(h(i,j)) is the minimum value in the gray-level co-occurrence matrix;

[0047] The expression for the dimension-reduced gray-level co-occurrence matrix y(i,j) is as follows:

[0048]

[0049] Calculate the contrast (con) of the gray-level co-occurrence matrix:

[0050]

[0051] Where y T (i,j) is the transpose of the gray-level co-occurrence matrix, expressed as follows:

[0052]

[0053] The characteristic parameters of the transformer bushing oscillation signal are obtained, and the expression for the characteristic parameters of the oscillation signal s1 is as follows:

[0054]

[0055] The characteristic parameter expressions for the oscillating wave signal s2 are as follows:

[0056]

[0057] Step 4: Assess the moisture level of the capacitor core in the bushing.

[0058] Calculate the moisture factor of the bushing capacitor core:

[0059]

[0060] The formula for calculating the evaluation coefficient ε of the moisture content of the bushing capacitor core is as follows:

[0061]

[0062] Where η e η is the moisture factor when the casing has no moisture defects. b The moisture factor when the casing has moisture defects;

[0063] When the evaluation coefficient is 0 < ε < 4.2, it indicates that the bushing capacitor core has no obvious moisture defects;

[0064] When the evaluation coefficient ε≥4.2, it indicates that the core of the bushing capacitor is severely damp and needs to be repaired or replaced.

Claims

1. A method for assessing moisture absorption of bushing capacitor cores based on high-frequency resonant excitation, characterized in that, The steps are as follows: Step 1: Building a high-frequency resonant excitation bushing capacitor core test platform A test platform for a bushing capacitor core under high-frequency resonant excitation is constructed, which mainly includes a high-voltage DC power supply (1), a protection resistor (2), a high-frequency switch (3), an adjustable inductor (4), a control switch 1 (5), a control switch 2 (6), and a bushing (7). The bushing (7) includes a capacitor core (8), a screen end (9), and a test terminal (10). The two ends of the bushing (7) are connected to a capacitor tester (11), and the screen end (9) and the test terminal (10) are connected to a signal tester (12). The high-voltage DC power supply (1), the protection resistor (2), and the adjustable inductor (4) are connected in series to the control switch 1 (5) and the control switch 2 (6). The high-frequency switch (3) is led out from the middle of the protection resistor (2) and the adjustable inductor (4). The high-voltage DC power supply (1) and the high-frequency switch (3) are both grounded. Step 2: Obtain the bushing signal under high-frequency resonant excitation (1) The capacitance C of the bushing is obtained by the capacitance tester. The inductance at the resonant frequency of 1000K is calculated according to formula (1) and recorded as L. The adjustable inductor (4) is adjusted to L. (2) Test the bushing signal under high frequency resonance excitation. At the start of the test, the high frequency switch (3) is in the open state, the control switch 1 (5) is in the closed state, the control switch 2 (6) is in the open state, the high voltage DC power supply (1) and the high frequency switch (3) are both grounded, the high voltage DC power supply (1) inputs DC voltage to the bushing (7) through the protection resistor (2) and the adjustable inductor (4) and the control switch 1 (5), close the high frequency switch (3), and the signal tester (12) reads the high frequency resonance signal s1 of the bushing end screen from the end screen end (9); open the high frequency switch (3), the control switch 1 (5) is in the open state, the control switch 2 (6) is in the closed state, the high voltage DC power supply (1) inputs DC voltage to the bushing (7) through the protection resistor (2) and the adjustable inductor (4) and the control switch 2 (6), close the high frequency switch (3), and the signal tester (12) reads the high frequency resonance signal s2 of the bushing test end from the test terminal (10); Step 3: Extracting the characteristic parameters of the bushing signal under high-frequency resonant excitation The oscillation wave signal s1 read from the end screen of the bushing and the oscillation wave signal s2 read from the test end of the bushing are converted into a time-frequency graph by the STFT short-time Fourier transform. The window function of the short-time Fourier transform is the Hamming window. Then the time-frequency graph is grayscaled to form a gray-level co-occurrence matrix x(i,j) with a size of n×n. The gray-level co-occurrence matrix is ​​reduced in dimension to obtain matrix y(i,j), calculated as follows: i,j are the coordinates of the co-occurrence matrix, λ is the dimensionality reduction factor related to the dimension of the co-occurrence matrix, with a value of 2, max(h(i,j)) is the maximum value in the gray-level co-occurrence matrix, and min(h(i,j)) is the minimum value in the gray-level co-occurrence matrix; The expression for the dimension-reduced gray-level co-occurrence matrix y(i,j) is as follows: Calculate the contrast (con) of the gray-level co-occurrence matrix: Where y T (i,j) is the transpose of the gray-level co-occurrence matrix, expressed as follows: The characteristic parameters of the transformer bushing oscillation wave signal are obtained, and the expression for the characteristic parameters of the oscillation wave signal s1 is as follows: The characteristic parameter expressions for the oscillating wave signal s2 are as follows: Step 4: Assess the moisture level of the capacitor core in the bushing. Calculate the moisture factor of the bushing capacitor core: The formula for calculating the evaluation coefficient ε of the moisture content of the bushing capacitor core is as follows: Where η e η is the moisture factor when the casing has no moisture defects. b The moisture factor when the casing has moisture defects; When the evaluation coefficient is 0 < ε < 4.2, it indicates that the bushing capacitor core has no obvious moisture defects; When the evaluation coefficient ε≥4.2, it indicates that the core of the bushing capacitor is severely damp and needs to be repaired or replaced.

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