High-voltage cable typical insulation defect live detection method based on transfer function

Abstract

The invention discloses a high-voltage cable typical insulation defect live detection method based on a transfer function, and belongs to the technical field of power equipment detection, and the method comprises the following steps: S1, determining a suspicious fault point on a to-be-detected cable, and setting a signal coupling device in a preset range of the suspicious fault point; s2, injecting a sweep frequency signal into the head section of the to-be-detected cable by using a signal coupling device, and receiving the sweep frequency signal at the tail section of the to-be-detected cable; s3, fitting the received sweep frequency signal, and constructing a transfer function; and S4, determining a fault judgment result of the to-be-detected cable according to the transfer function and the resonant structure. The method has a good effect on the power-frequency high-voltage cable, and because the frequency of the voltage is low, the frequency of generated multiple harmonic waves is also low, an effective measurement result can be obtained by observing a high-frequency signal without worrying about excessive influence of the high-voltage signal and the harmonic waves thereof on the measurement result.

Description

Technical Field

[0001] This invention belongs to the field of power equipment testing technology, specifically relating to a live-line detection method for typical insulation defects in high-voltage cables based on transfer functions. Background Technology

[0002] Cross-linked polyethylene (XLPE) cables have been widely used in power systems both domestically and internationally due to their excellent performance, simple manufacturing process, and easy installation. However, with the increasing coverage of power cables, not only have the cable lines become more complex, but cables put into operation earlier are also entering their aging phase. To ensure the stable operation of cables in power systems, it is necessary to regularly inspect and evaluate the insulation condition of power cables. Current insulation condition inspection methods still have many shortcomings: repeated tests on cables with different insulation conditions lack specificity; multiple preventative tests can easily damage cables with good insulation, thus accelerating insulation aging; offline insulation condition inspection methods lack real-time capability, making it difficult to promptly grasp the insulation condition of power cables. Therefore, it is necessary to propose an online insulation condition inspection method for power cables. Summary of the Invention

[0003] To address the above problems, this invention proposes a live-line detection method for typical insulation defects in high-voltage cables based on transfer functions.

[0004] The technical solution of this invention is: a live-line detection method for typical insulation defects in high-voltage cables based on transfer function, comprising the following steps:

[0005] S1. Identify suspected fault points on the cable to be tested, and set up signal coupling devices within a preset range of the suspected fault points;

[0006] S2. Inject a sweep frequency signal into the first section of the cable under test using a signal coupling device, and receive the sweep frequency signal at the last section of the cable under test.

[0007] S3. After fitting the received sweep frequency signal, construct the transfer function;

[0008] S4. Based on the transfer function and resonant structure, determine the fault diagnosis result of the cable under test.

[0009] Furthermore, in S2, the formula for calculating the capacitance C of the signal coupling device is:

[0010]

[0011] In the formula, h represents the length of the cylinder, b represents the radius of the outer cylinder, a represents the radius of the inner cylinder, ε0 represents the vacuum permittivity, and ε r This represents the relative permittivity of the material.

[0012] The vacuum permittivity is approximately 8.85 × 10⁻⁶. -12 F / m.

[0013] Furthermore, S3 includes the following sub-steps:

[0014] S31. Perform Fast Fourier Decomposition on the received sweep frequency signal to obtain the sweep frequency signal in the frequency domain.

[0015] S32. Fit the frequency sweep signal in the frequency domain using a double exponential function;

[0016] S33. Based on the fitted swept frequency signal, construct the transfer function.

[0017] Furthermore, in S32, the expression for the double exponential function y is:

[0018] y = a·e bx +d·e ex +c;

[0019] In the formula, a represents the first compensation number of the compensation system, b represents the second compensation number of the compensation system, c represents the third compensation number of the compensation system, d represents the fourth compensation number of the compensation system, x represents the frequency, and e represents the exponent.

[0020] Furthermore, in S33, the expression for the transfer function H(ω) is:

[0021]

[0022] In the formula, U o (ω) represents the frequency sweep signal in the frequency domain, C represents a constant, and K represents a constant.

[0023] Furthermore, S4 includes the following sub-steps:

[0024] S41. Based on the characteristic amplitude-frequency diagram of the transfer function, determine the resonant structure of the signal transmission path;

[0025] S42. By using the inductance matrix and capacitance matrix of the resonant structure and the voltage-current coupling equation, determine the waveform amplitude difference and the characteristic frequency difference of the peak value between the good cable and the cable under test.

[0026] S43. Based on the difference in waveform amplitude and the difference in characteristic frequency of peak values ​​between the good cable and the cable under test, determine the fault judgment result of the cable under test.

[0027] Furthermore, in S41, the resonant structure includes several resonant circuits.

[0028] Furthermore, in S42, the expression for the inductance matrix L is:

[0029]

[0030] In the formula, L1 represents the self-inductance of the first loop, L2 represents the self-inductance of the second loop, and L... n Let M represent the self-inductance of the nth circuit. 12 M represents the mutual inductance between the first and second loops. 21 M represents the mutual inductance between the second loop and the first loop. 1n M represents the mutual inductance between the 1st loop and the nth loop. 2n M represents the mutual inductance between the second loop and the nth loop. n1 M represents the mutual inductance between the nth loop and the 1st loop. n2 This represents the mutual inductance between the nth loop and the second loop;

[0031] In S42, the expression for the capacitance matrix C is:

[0032]

[0033] In the formula, C1 represents the capacitance of the first circuit, C2 represents the capacitance of the second circuit, and C... n This represents the capacitance of the nth circuit.

[0034] Furthermore, in S42, the method for determining the characteristic frequency of the peak value of the good cable and the cable under test is as follows: construct a voltage-current coupling equation, solve the voltage-current coupling equation to obtain the eigenvalue solution equation, and take the eigenvalue of the eigenvalue solution equation as the characteristic frequency of the peak value.

[0035] Furthermore, the expression for solving the eigenvalue equation is:

[0036] det(L -1 C -1 -ω 2 I) = 0;

[0037] In the formula, L represents the inductance matrix, C represents the capacitance matrix, ω represents the angular frequency, and I represents the identity matrix.

[0038] The beneficial effects of this invention are:

[0039] (1) This invention uses coupling capacitors to input and receive signals from high-voltage cables. By using the coupling between the capacitors before and after the signal, the signal transmission results inside the cable are obtained. The changes in admittance of the signal during the transmission of the high-voltage cable are obtained, and the obtained signal is then processed accordingly. The corresponding transfer function and amplitude-frequency diagram are extracted from the signal, which realizes the judgment of insulation defects in high-voltage cables. On-site personnel can perform online operations without disassembling and repairing the cable. They only need to install two coupling capacitors at both ends of the potentially faulty parts to perform the detection, which greatly improves the efficiency of detection and repair.

[0040] (2) The present invention effectively controls the detection range by controlling the placement of the input and receiving capacitors, which not only improves the detection efficiency, but also avoids excessive signal attenuation after long-distance transmission.

[0041] (3) After receiving the signal, the present invention processes it to convert the signal in the time domain into the signal in the frequency domain, thereby obtaining the characteristics of the signal in the frequency domain. The spectrum diagram is more obvious than the characteristics in the time domain and is easier to calculate and identify. At the same time, the signal is compensated for high frequency. This is because high frequency signals will be attenuated when they are transmitted through the equipment. Its attenuation function is similar to a double exponential function. After compensation, the signal is converted into a transfer function characteristic amplitude-frequency diagram. This amplitude-frequency diagram is not affected by the transmission equipment compared to the one before compensation. This eliminates the variables in this regard, and the result is more accurate and easier to identify.

[0042] (4) The present invention is effective for power frequency high voltage cables. Since the voltage frequency is low, the frequency of the multiple harmonics generated is also low. Effective measurement results can be obtained by observing high frequency signals without worrying about the high voltage signal and its harmonics having too much influence on the measurement results. Attached Figure Description

[0043] Figure 1 The flowchart shows a typical live-line detection method for insulation defects in high-voltage cables based on transfer functions.

[0044] Figure 2 This is a schematic diagram of a live-line detection method for typical insulation defects in high-voltage cables based on transfer functions.

[0045] Figure 3 This is a diagram of the distributed parameter model of the single-phase cable used in this invention;

[0046] Figure 4 This is a diagram of the frequency domain cable distribution parameter model used in this invention;

[0047] Figure 5 This is a schematic diagram of signal transmission attenuation.

[0048] Figure 6 This is a schematic diagram before signal compensation.

[0049] Figure 7 This is a schematic diagram after signal compensation.

[0050] Figure 8 This is a schematic diagram of the cross-section of a high-voltage cable;

[0051] Figure 9 A comparison chart of defective cable inspection data;

[0052] Figure 10 A comparison chart of good cable testing data. Detailed Implementation

[0053] The embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0054] like Figure 1 As shown, this invention provides a live-line detection method for typical insulation defects in high-voltage cables based on transfer functions, comprising the following steps:

[0055] S1. Identify suspected fault points on the cable to be tested, and set up signal coupling devices within a preset range of the suspected fault points;

[0056] S2. Inject a sweep frequency signal into the first section of the cable under test using a signal coupling device, and receive the sweep frequency signal at the last section of the cable under test.

[0057] S3. After fitting the received sweep frequency signal, construct the transfer function;

[0058] S4. Based on the transfer function and resonant structure, determine the fault diagnosis result of the cable under test.

[0059] In field measurements of high-voltage cables, signals are injected and received through coupling capacitors wrapped around the outside of the cable. When the buffer layer or insulation layer of the high-voltage cable becomes damp, corroded, or burned, the volume resistivity and relative permittivity change, resulting in impedance discontinuities in the cable's transmission network. According to transmission line theory, signals will undergo refraction and reflection when passing through impedance discontinuities in the cable. By processing the received signal, we can obtain the corresponding amplitude-frequency diagram and transfer function.

[0060] In this embodiment of the invention, the signal injection and receiving module of the method of the present invention is as follows: Figure 2 As shown, it includes coupling capacitors, a signal generator, a signal acquisition unit, and a computer. Specifically:

[0061] A pair of coupling capacitors are used for the injection and reception of transmitted signals within the cable. The signal is transmitted from the coupling capacitor to the internal metal sheath and copper core of the cable, and then transmitted out from the receiving end through the coupling capacitor, completing one transmission process of the signal within the cable.

[0062] A signal generator is used to generate function and frequency sweep signals.

[0063] A signal acquisition device is used to receive the acquired transmission signals and transmit them to a personal computer for processing and transformation to obtain the required charts and data.

[0064] Computers are used to process received signals, converting time-domain signals into corresponding frequency-domain signals, compensating for attenuated frequency-domain signals, and obtaining the corresponding transfer function. Mathematical processing is performed on different signals to obtain their correlation data.

[0065] As a signal transmission line, power cables utilize transmission line theory. Under AC signals, power cables can be modeled as distributed parameter cables to describe the signal transmission characteristics. This distributed parameter model plays a crucial theoretical role in analyzing the transmission process of voltage signals. Figure 3 This is a distributed parameter model diagram of a single-phase cable. In the diagram, R, L, G, and C represent the resistance, inductance, conductance, and capacitance per unit length of the cable, respectively. u(x,t) represents the voltage of the cable at a distance x and time t, i(x,t) represents the current inside the cable at a distance x and time t, Δx represents a unit length distance, u(x+Δx,t) represents the voltage of the cable at a distance x+Δx and time t, and i(x+Δx,t) represents the current inside the cable at a distance x+Δx and time t.

[0066] For cables undergoing overall insulation aging, their insulation parameters will change significantly, thus affecting the voltage transfer characteristics in the frequency domain. The cable model in the frequency domain is as follows: Figure 4 As shown. Z c (f) represents the impedance per unit length of the conductor layer in the frequency domain, Y in (f) represents the admittance per unit length of the insulating layer in the frequency domain, Z s (f) represents the impedance per unit length of the metal shielding layer in the frequency domain, I C (x) represents the current inside the cable at a distance x, I S (x) represents the current in the metal shield of the cable at a distance x, U C (x) represents the voltage inside the cable at a distance x, U S (x) represents the voltage inside the metal shield of the cable at a distance x, dx, dI, and dU represent the changes in current and voltage per unit length and per unit length, respectively, and Z load R represents the load impedance. g This indicates the grounding impedance.

[0067] According to Kirchhoff's current and voltage laws, a relationship can be established with... Figure 4 The corresponding differential equation is as follows:

[0068]

[0069] In the formula, This indicates the current in the cable conductor. This represents the current in the metal shielding layer. Indicates the voltage of the cable conductor to ground. Y represents the voltage of the cable's metallic shield to ground. in Z represents the input impedance of the cable. c Z represents the impedance per unit length of the conductor layer. s This represents the impedance per unit length of the metal shielding layer;

[0070]

[0071] In the formula, e represents the exponent, γ represents the propagation coefficient, x represents the transmission distance, and C1, C2, C3, and C4 are coefficients determined by the boundary conditions of the second-order differential equation. Assuming the total length of the cable is l, the voltages at both ends of the cable can be obtained as follows:

[0072]

[0073] In the formula, This indicates a distance of 0, which is the magnitude of the voltage at the excitation terminal. This indicates the magnitude of the voltage output at a location at a distance of l.

[0074] That is, the voltage transfer function H of the cable in the frequency domain is:

[0075]

[0076] As can be seen from the above formula, the transfer function changes with the changes of l and γ in the formula. The voltage transfer function in the frequency domain is closely related to the cable length and the insulation impedance. Whether it increases or decreases needs to be calculated in practice at a certain frequency and cable length. This invention is based on the above relationship between the cable and the change of the transfer function.

[0077] In S1, since the purpose of this invention is to detect insulation defects in high-voltage cables, it is necessary to locate a suspected fault point on the cable being tested. The suspected fault point can be identified by observing the cable's appearance, temperature, noise, or by relying on existing experience with cable faults to initially pinpoint the defect. Signal coupling devices are then installed before and after the fault point. Figure 2The diagram shows a simulation of the detection of the present invention. Taking the measurement of a 110KV high-voltage cable as an example, it is necessary to set up a signal receiving and acquisition coupling device with a front and rear interval of 1m around the fault point where there may be defects. Here, a cylindrical coupling capacitor made of a 10cm wide and 0.5mm thick copper sheet is used.

[0078] In S2, a function signal generator is used to directly inject a signal by connecting the input coupling capacitor via the probes. The signal used here is a 0-50MHz sinusoidal sweep signal with a sweep period of 2ms. Figure 5 As shown. Injecting this signal from the beginning of the section of the cable to be tested will allow a corresponding signal to be received at the end of that section.

[0079] In S2, when the signal is transmitted from the fault point to the 1m receiver, an oscilloscope is connected to the signal receiving coupling capacitor to acquire the coupled 0-50MHz sinusoidal sweep frequency signal. Because the sweep period is short, multiple sets of repetitive signals can be acquired at once. Processing these repetitive signals effectively avoids errors caused by background noise or other factors due to insufficient sample size.

[0080] In this embodiment of the invention, in S2, since the method used in this invention is a capacitive coupling method, the signal is coupled from the externally installed capacitor into the internal metal shielding layer and copper core, and then transmitted to the other end of the cable. During this process, due to defects in some parts of the cable, the magnitude of the coupling capacitance will change with variations in the buffer layer, semiconductive layer, and other dielectric materials. It is easy to see that: Where S represents the area of ​​the two plates facing each other, d represents the distance between the two plates, and k represents the electrostatic constant, approximately 8.99 × 10⁻⁶. 9 N·m 2 / C 2 The dielectric constant ε is closely related to the state of the medium; therefore, the presence of defects in a section can be effectively determined by the signal transmission results. For high-voltage cables, a hollow cylinder can be used to simulate them. Figure 8 The cross-sectional model of high-voltage cables is explained in the text. Specifically:

[0081] Cable core: The cable core is made of high conductivity material to facilitate more efficient transmission of electrical energy. At present, the most commonly used materials for cable core are copper or aluminum metal, which have the advantages of high conductivity, high strength and low cost.

[0082] Conductor shielding layer: The inner semiconducting layer fills the air gap between the cable core and the insulation layer, ensuring a smooth surface for the core. Good contact between the inner semiconductor layer and the cable core ensures a uniform electric field distribution and prevents partial discharge between the cable core and the insulation layer.

[0083] Insulation layer: To ensure insulation capability, the insulation layer is usually quite thick. It can isolate the high-voltage current in the cable core from external electrical connections, ensuring the safe operation of the cable.

[0084] Insulating shielding layer: The outer semiconductor layer and the inner semiconductor layer are made of the same material and are at the same potential as the metal sheath, which can prevent partial discharge of the metal sheath due to cracks or defects on the cable surface.

[0085] Water-blocking buffer layer: Composed of two layers of semi-conductive fibers with a layer of waterproof powder sandwiched in between. It provides good electrical performance between the insulating shield and the metal sheath, while also offering waterproof and thermal insulation effects.

[0086] Corrugated aluminum sheath: Corrugated aluminum sheath has a spiral structure and is made of low-cost aluminum welded together. Its main function is to improve the mechanical strength of power cables and prevent moisture or humidity from the external environment from entering the cable.

[0087] Outer sheath: The outer sheath is in direct contact with the external environment where the cable is laid, and is used to prevent harsh external environments from affecting the internal structure of the cable. The material of the outer sheath is generally polyethylene and polyvinyl chloride.

[0088] The signal is coupled to the inner shielding layer through the outer sheath. The signal sensed by the shielding layer can also be coupled to the inner cable core. The formula for calculating the capacitance C of the signal coupling device is:

[0089]

[0090] In the formula, h represents the length of the cylinder, b represents the radius of the outer cylinder, a represents the radius of the inner cylinder, ε0 represents the vacuum permittivity, and ε r This represents the relative permittivity of the material.

[0091] The vacuum permittivity is approximately 8.85 × 10⁻⁶. -12 F / m.

[0092] In this embodiment of the invention, S3 includes the following sub-steps:

[0093] S31. Perform Fast Fourier Decomposition on the received sweep frequency signal to obtain the sweep frequency signal in the frequency domain.

[0094] S32. Fit the frequency sweep signal in the frequency domain using a double exponential function;

[0095] S33. Based on the fitted swept frequency signal, construct the transfer function.

[0096] In S31, after receiving the frequency sweep signal, the signal is transmitted to the computer, where mathematical software performs FFT (Fast Fourier Transform) processing. FFT is an algorithm used to quickly calculate the Discrete Fourier Transform (DFT). DFT can transform a signal from the time (or space) domain to the frequency domain, helping to analyze the frequency components of the signal. For a given signal x[n] of length N, directly calculating the DFT requires N... 2 The computational workload is large. The FFT greatly reduces the computational workload by dividing the signal into two halves: even points and odd points, calculating their DFTs separately, and then combining the results.

[0097] After obtaining the frequency sweep signal in the frequency domain, the transfer function needs to be calculated to determine if there is a potential fault in the cable segment. Therefore, compensation is required for the transformed signal. Through reviewing relevant materials and conducting actual measurements, it was found that the high-frequency signal emitted by the signal generator experiences amplitude attenuation after passing through the probe and acquisition card. For example... Figure 5 As shown in the figure, the output signal received by the acquisition card (the signal is a 0-50MHz sweep frequency signal with a peak value of 2.5V) shows a decreasing trend as the frequency increases.

[0098] In this embodiment of the invention, in S32, the expression for the double exponential function y is:

[0099] y = a·e bx +d·e ex +c;

[0100] In the formula, a represents the first compensation number of the compensation system, b represents the second compensation number, c represents the third compensation number, d represents the fourth compensation number, x represents the frequency, and e represents the exponent. a, b, c, and d are constants, and their specific values ​​vary with the system.

[0101] In this embodiment of the invention, in step S33, after obtaining the relevant parameters of the function through fitting, the function is inverted to obtain the compensated waveform, specifically as follows: Figure 7 As shown.

[0102] The transfer function formula in the circuit is:

[0103]

[0104] In the formula, Y(ω) represents the output response and U(ω) represents the input response.

[0105] After converting the variables in the formula into input and output signals respectively, the formula can be transformed into...

[0106]

[0107] In the formula, U i (ω) represents the input signal in the frequency domain. The compensated signal can be considered as an input signal with no attenuation, and when the input signal has no attenuation, the transfer function can be rewritten. The expression for the transfer function H(ω) is:

[0108]

[0109] In the formula, U o (ω) represents the frequency sweep signal in the frequency domain, C represents a constant, and K represents a constant. Here, C is a constant, representing a signal with a constant output voltage amplitude of C from 0 to 50 MHz. Rewriting 1 / C as K yields the final expression of the transfer function, which is the output signal U. o Multiplying (ω) by a constant K, the transfer function can now fully express the various characteristics of the output signal waveform in the frequency domain. By observing and comparing the transfer functions, the required information can be obtained.

[0110] In this embodiment of the invention, S4 includes the following sub-steps:

[0111] S41. Based on the characteristic amplitude-frequency diagram of the transfer function, determine the resonant structure of the signal transmission path;

[0112] S42. By using the inductance matrix and capacitance matrix of the resonant structure and the voltage-current coupling equation, determine the waveform amplitude difference and the characteristic frequency difference of the peak value between the good cable and the cable under test.

[0113] S43. Based on the difference in waveform amplitude and the difference in characteristic frequency of peak values ​​between the good cable and the cable under test, determine the fault judgment result of the cable under test.

[0114] In this embodiment of the invention, in S41, the resonant structure includes a plurality of resonant circuits.

[0115] In this embodiment of the invention, in S42, the expression for the inductance matrix L is:

[0116]

[0117] In the formula, L1 represents the self-inductance of the first loop, L2 represents the self-inductance of the second loop, and L... n Let M represent the self-inductance of the nth circuit. 12 M represents the mutual inductance between the first and second loops. 21 M represents the mutual inductance between the second loop and the first loop. 1n M represents the mutual inductance between the 1st loop and the nth loop. 2n M represents the mutual inductance between the second loop and the nth loop. n1 M represents the mutual inductance between the nth loop and the 1st loop. n2This represents the mutual inductance between the nth loop and the second loop;

[0118] In S42, the expression for the capacitance matrix C is:

[0119]

[0120] In the formula, C1 represents the capacitance of the first circuit, C2 represents the capacitance of the second circuit, and C... n This represents the capacitance of the nth circuit.

[0121] In this embodiment of the invention, in S42, the method for determining the characteristic frequency of the peak value of the good cable and the cable to be tested is as follows: constructing a voltage-current coupling equation, solving the voltage-current coupling equation to obtain the eigenvalue solution equation, and taking the eigenvalue of the eigenvalue solution equation as the characteristic frequency of the peak value.

[0122] In this embodiment of the invention, the expression for the eigenvalue solving equation is:

[0123] det(L -1 C -1 -ω 2 I) = 0;

[0124] In the formula, L represents the inductance matrix, C represents the capacitance matrix, ω represents the angular frequency, and I represents the identity matrix.

[0125] After obtaining the transfer function graph, by comparing the waveform amplitude differences and peak frequency differences between a good cable and a potentially faulty cable, the presence of a fault in the cable can be determined. This can be achieved through observation. Figure 9 and Figure 10 It was found that the peak value of the output signal at 5MHz of the good cable was relatively small under multiple measurements, which can be attributed to the influence of background noise. However, the peak value of the output signal at 5MHz of the good cable and the cable with suspected faults differed significantly, reaching 120mV.

[0126] For different cable conditions, the characteristic frequency of the peak value is different, and an explanation is given for the different characteristic frequencies of the peak value appearing in the transfer function.

[0127] Depend on Figure 6 and Figure 7 The measured graph shows peaks at different characteristic frequencies after signal transmission, indicating that the signal transmission path forms a complex resonant structure, consisting of multiple resonant modes rather than a single resonant frequency. The magnitudes of these peaks and characteristic frequencies are related to the capacitance (L) and capacitance (C) of various parts of the entire signal transmission structure. Therefore, by observing the changes in these peak magnitudes and characteristic frequencies, it is possible to effectively determine a series of parameter changes occurring within the cable, thereby detecting related defects.

[0128] This process can be represented through a series of matrix operations. Assume there are n resonant circuits, each consisting of an inductor L. i and capacitor C i Composition. They are interconnected through mutual inductance M ij Mutual coupling (i≠j), mutual inductance represents the magnetic field influence of loop i on loop j.

[0129] By solving the voltage-current coupling equation of the circuit, the solution to the resonant frequency can be simplified into an eigenvalue equation. By converting the eigenvalues ​​of the eigenvalue equation into frequencies, the corresponding characteristic frequencies of the system can be obtained.

[0130] Those skilled in the art will recognize that the embodiments described herein are intended to help the reader understand the principles of the invention, and should be understood that the scope of protection of the invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific modifications and combinations based on the technical teachings disclosed in this invention without departing from the spirit of the invention, and these modifications and combinations are still within the scope of protection of this invention.

Claims

1. A live-line detection method for typical insulation defects in high-voltage cables based on transfer function, characterized in that, Includes the following steps: S1. Identify suspected fault points on the cable to be tested, and set up signal coupling devices within a preset range of the suspected fault points; S2. Inject a sweep frequency signal into the first section of the cable under test using a signal coupling device, and receive the sweep frequency signal at the last section of the cable under test. S3. After fitting the received sweep frequency signal, construct the transfer function; S4. Based on the transfer function and resonant structure, determine the fault diagnosis result of the cable under test.

2. The live-line detection method for typical insulation defects in high-voltage cables based on transfer function according to claim 1, characterized in that, In S2, the formula for calculating the capacitance C of the signal coupling device is: In the formula, h represents the length of the cylinder, b represents the radius of the outer cylinder, a represents the radius of the inner cylinder, ε0 represents the vacuum permittivity, and ε r This represents the relative permittivity of the material.

3. The live-line detection method for typical insulation defects in high-voltage cables based on transfer function according to claim 1, characterized in that, S3 includes the following sub-steps: S31. Perform Fast Fourier Decomposition on the received sweep frequency signal to obtain the sweep frequency signal in the frequency domain. S32. Fit the frequency sweep signal in the frequency domain using a double exponential function; S33. Construct the transfer function based on the fitted swept frequency signal.

4. The live-line detection method for typical insulation defects in high-voltage cables based on transfer function according to claim 3, characterized in that, In S32, the expression for the double exponential function y is: y=a·e bx +d·e ex +c; In the formula, a represents the first compensation number of the compensation system, b represents the second compensation number of the compensation system, c represents the third compensation number of the compensation system, d represents the fourth compensation number of the compensation system, x represents the frequency, and e represents the exponent.

5. The live-line detection method for typical insulation defects in high-voltage cables based on transfer function according to claim 3, characterized in that, In S33, the expression for the transfer function H(ω) is: In the formula, U o (ω) represents the frequency sweep signal in the frequency domain, C represents a constant, and K represents a constant.

6. The live-line detection method for typical insulation defects in high-voltage cables based on transfer function according to claim 1, characterized in that, S4 includes the following sub-steps: S41. Based on the characteristic amplitude-frequency diagram of the transfer function, determine the resonant structure of the signal transmission path; S42. By using the inductance matrix and capacitance matrix of the resonant structure and the voltage-current coupling equation, determine the waveform amplitude difference and the characteristic frequency difference of the peak value between the good cable and the cable under test. S43. Based on the difference in waveform amplitude and the difference in characteristic frequency of peak values ​​between the good cable and the cable under test, determine the fault judgment result of the cable under test.

7. The live-line detection method for typical insulation defects in high-voltage cables based on transfer function according to claim 6, characterized in that, In S41, the resonant structure includes several resonant circuits.

8. The live-line detection method for typical insulation defects in high-voltage cables based on transfer function according to claim 6, characterized in that, In S42, the expression for the inductance matrix L is: In the formula, L1 represents the self-inductance of the first loop, L2 represents the self-inductance of the second loop, and L... n Let M represent the self-inductance of the nth circuit. 12 M represents the mutual inductance between the first and second loops. 21 M represents the mutual inductance between the second loop and the first loop. 1n M represents the mutual inductance between the 1st loop and the nth loop. 2n M represents the mutual inductance between the second loop and the nth loop. n1 M represents the mutual inductance between the nth loop and the 1st loop. n2 This represents the mutual inductance between the nth loop and the second loop; In S42, the expression for the capacitance matrix C is: In the formula, C1 represents the capacitance of the first circuit, C2 represents the capacitance of the second circuit, and C... n This represents the capacitance of the nth circuit.

9. The live-line detection method for typical insulation defects in high-voltage cables based on transfer function according to claim 6, characterized in that, In step S42, the method for determining the characteristic frequency of the peak value of the good cable and the cable under test is as follows: construct a voltage-current coupling equation, solve the voltage-current coupling equation to obtain the eigenvalue solution equation, and take the eigenvalue of the eigenvalue solution equation as the characteristic frequency of the peak value.

10. The live-line detection method for typical insulation defects in high-voltage cables based on transfer function according to claim 9, characterized in that, The expression for the eigenvalue solution equation is: it(L -1 C -1 -ω 2 I)=0; In the formula, L represents the inductance matrix, C represents the capacitance matrix, ω represents the angular frequency, and I represents the identity matrix.

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