Cable fault identification method based on bipolar pulse square wave excitation

Abstract

The invention relates to the technical field of electric power engineering, discloses a cable fault identification method based on bipolar pulse square wave excitation, and solves the problems that in the prior art, when a cable fault point is identified, sine waves are used for excitation, the frequency is single, and detail changes are difficult to capture. The bipolar pulse square wave is used as the excitation signal, compared with sine wave and square wave signals, the bipolar pulse square wave has rich harmonic component characteristics, the high-frequency impedance characteristic of the shielding cable in the broadband range can be effectively obtained, and compared with unipolar pulse square waves, the bipolar pulse square wave has the advantage of being wider in harmonic distribution range. After the excitation signal is injected into the shielded cable, the response signal of the shielded cable is collected to obtain the real impedance spectrum, and the real impedance spectrum and the standard impedance spectrum generated based on the transmission line model are compared and analyzed to identify and position the fault point, so that the cable fault point can be identified in a wider frequency range; and detail change capture of the cable under excitation is realized.

Description

Technical Field

[0001] This invention relates to the field of power engineering technology, and in particular to a cable fault identification method based on bipolar pulse square wave excitation. Background Technology

[0002] Cables, acting as the lifeblood of power transmission and distribution networks, play an indispensable role in the normal operation of power systems, responsible for transmitting electrical energy and signals. Cross-linked polyethylene (XLPE) cables, as a type of shielded cable, are widely used in my country's 10-220kV power transmission and distribution networks due to their excellent dielectric and physicochemical properties. XLPE cables are typically designed for a lifespan of 20-30 years. In urban power grids, XLPE cables are usually laid in underground cable trenches, underground pipes, within walls, or directly buried underground. In these complex operating environments, XLPE cables are prone to insulation defects due to various factors such as temperature, humidity, mechanical force, and chemical corrosion. Furthermore, the insulation performance of cables in operation often declines rapidly due to localized aging or damage, reducing their lifespan and affecting the reliability of the power system. Therefore, the detection of insulation defects and faults in cables is crucial.

[0003] Currently, broadband impedance spectroscopy is commonly used for fault point detection. Its basic principle is to use a sine wave as the input signal and use an impedance analyzer to test at the beginning of the cable, obtaining the input impedance at different frequency points one by one, and obtaining the input impedance spectrum that varies with frequency. By analyzing the spectrum, relevant characteristic parameters of the cable's operating status can be extracted, the high-frequency transmission characteristics of the distribution network cable can be studied, and thus insulation defect diagnosis can be achieved.

[0004] However, this method has many limitations when analyzing high-frequency impedance characteristics due to the use of sinusoidal excitation. For example, the sinusoidal signal has a single frequency, making it difficult to comprehensively reflect the characteristic changes of the cable over a wide frequency range. Moreover, in the high-frequency band, the skin effect and proximity effect are amplified, and the effective resistance and inductance of the conductors inside the cable change significantly, making it difficult for traditional sinusoidal excitation methods to accurately capture these changes.

[0005] Therefore, how to identify cable fault points over a wider frequency range and capture the detailed changes in cables under excitation is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0006] The purpose of this invention is to solve the problem that the current technology of using sine waves for excitation in cable fault point identification has a single frequency and is difficult to capture detailed changes. Therefore, this invention provides a cable fault identification method based on bipolar pulse square wave excitation, which can identify cable fault points in a wider frequency range and capture the detailed changes of the cable under excitation.

[0007] To address the aforementioned technical problems, this invention provides a cable fault identification method based on bipolar pulse square wave excitation, comprising:

[0008] After a bipolar pulse square wave excitation signal is injected into the shielded cable, the response signal of the shielded cable under excitation is acquired; the response signal includes a voltage signal and a current signal.

[0009] The time-domain response signal is converted into a frequency-domain signal to calculate the impedance value of the shielded cable at different frequencies;

[0010] The amplitude-frequency and phase-frequency characteristics are calculated based on the impedance values ​​to obtain the true impedance spectrum.

[0011] The actual impedance spectrum is compared and analyzed with the standard impedance spectrum to identify and locate the fault point.

[0012] Preferably, the bipolar pulse square wave excitation signal satisfies odd symmetry characteristics.

[0013] Preferably, the formula for obtaining the standard impedance spectrum includes:

[0014] A lossy transmission line model is constructed based on the hardware structure of the shielded cable;

[0015] Based on the aforementioned lossy transmission line model, a standard impedance spectrum is generated according to the following formula;

[0016] ;

[0017] in, For load impedance, The fundamental frequency of the bipolar pulse square wave excitation signal is... for Characteristic impedance at the subharmonic frequency. for The propagation coefficient of the subharmonic frequency, The amplitude of the bipolar pulse square wave excitation signal. Indicates frequency at Discrete components at the location, For the length of the shielded cable, For input impedance, It is a constant.

[0018] Preferably, the voltage signal acquisition device is a voltage sensor with a capacitive voltage divider structure; the current signal acquisition device is a current sensor designed using the Rogowski coil principle.

[0019] Preferably, the bipolar pulse square wave excitation signal injection shielded cable includes:

[0020] The bipolar pulse square wave excitation signal is amplified to a preset intensity by a power amplifier and then injected into the shielded cable using an isolation transformer.

[0021] Preferably, after the step of acquiring the response signal of the shielded cable under excitation, the method further includes:

[0022] The wavelet transform denoising algorithm is used to remove noise components from the response signal.

[0023] Preferred options also include:

[0024] The response signal is filtered using a Butterworth low-pass filter.

[0025] Preferably, before the step of calculating the amplitude-frequency characteristics and phase-frequency characteristics based on the impedance values ​​to obtain the true impedance spectrum, the method further includes:

[0026] The system noise in the response signal is subtracted by no-load calibration.

[0027] Preferred options also include:

[0028] Based on the variation of the real and imaginary parts of the impedance value with frequency, impedance frequency characteristic curves are plotted to analyze the resistance, inductance, and capacitance variation characteristics of the shielded cable in different frequency bands.

[0029] Preferably, the frequency of the bipolar pulse square wave excitation signal is 10MHz and the rise time is within 20ns.

[0030] This invention provides a cable fault identification method based on bipolar pulsed square wave excitation. Compared to current technologies that use sine waves for cable fault point identification, which suffer from limitations such as single-frequency excitation and difficulty in capturing detailed changes, this application uses a bipolar pulsed square wave excitation signal. Compared to a sine wave, the pulsed square wave excitation signal possesses rich harmonic components, effectively acquiring the high-frequency impedance characteristics of the shielded cable over a wide frequency range, thus improving the accuracy and comprehensiveness of the cable's high-frequency characteristic analysis. Furthermore, compared to a unipolar pulsed square wave excitation signal, the bipolar pulsed square wave excitation signal offers a wider harmonic distribution range and richer high-frequency harmonic components, allowing for impedance spectrum plotting over a broader frequency range after injection into the shielded cable. Using this technical solution, after the bipolar pulsed square wave excitation signal is injected into the shielded cable, the response signal of the shielded cable under excitation is collected to obtain the true impedance spectrum. By comparing and analyzing this spectrum with a standard impedance spectrum generated based on a transmission line model, fault points can be identified and located. This enables cable fault point identification over a wider frequency range and achieves the capture of detailed changes in the cable under excitation. Attached Figure Description

[0031] To more clearly illustrate the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 A flowchart of a cable fault identification method based on bipolar pulse square wave excitation provided for an embodiment of the present invention;

[0033] Figure 2 A schematic diagram of a bipolar pulse square wave excitation provided in an embodiment of the present invention;

[0034] Figure 3 A schematic diagram of a bipolar pulse square wave signal and its Fourier decomposition provided in an embodiment of the present invention;

[0035] Figure 4 A comparison diagram of impedance spectrum amplitude characteristics under simulation and actual measurement is provided for an embodiment of the present invention;

[0036] Figure 5 This is a comparison diagram of the phase characteristics of impedance spectrum under simulation and actual measurement provided for an embodiment of the present invention. Detailed Implementation

[0037] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the protection scope of the present invention.

[0038] The core of this invention is to provide a cable fault identification method based on bipolar pulse square wave excitation, which can identify cable fault points over a wider frequency range and capture the detailed changes of the cable under excitation.

[0039] To enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0040] Figure 1 A flowchart of a cable fault identification method based on bipolar pulse square wave excitation provided for embodiments of the present invention is shown below. Figure 1 As shown, the method includes:

[0041] S10: After the bipolar pulse square wave excitation signal is injected into the shielded cable, the response signal of the shielded cable under excitation is collected; the response signal includes voltage signal and current signal;

[0042] S11: Convert the time-domain response signal into a frequency-domain signal for calculating the impedance value of the shielded cable at different frequencies;

[0043] S12: Calculate the amplitude-frequency and phase-frequency characteristics based on the impedance value to obtain the true impedance spectrum;

[0044] S13: Compare and analyze the actual impedance spectrum with the standard impedance spectrum to identify and locate the fault point.

[0045] The cable fault identification method based on bipolar pulse square wave excitation provided in this application is used to identify and locate fault points such as partial discharge or insulation aging in shielded cables. The execution subject of this method can be a bipolar pulse square wave excitation cable fault identification device, which in specific implementations can be a high-frequency impedance analyzer. This device may specifically include a memory and a processor. The memory stores a computer program, and the processor executes the computer program to implement the steps of the bipolar pulse square wave excitation cable fault identification method provided in the above embodiments. In some embodiments, the bipolar pulse square wave excitation cable fault identification device may also include a display, touchscreen, or other human-computer interaction device. In specific implementations, the bipolar pulse square wave excitation cable fault identification device provided in this embodiment may include, but is not limited to, smartphones, tablets, laptops, or desktop computers.

[0046] Of course, it is understood that if the methods in the above embodiments are implemented as software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, all or part of the technical solutions of this application can be embodied in the form of a software product, which is stored in a storage medium and executes all or part of the steps of the methods described in the various embodiments of this application.

[0047] This application identifies fault points by injecting a pulsed square wave excitation signal into the shielded cable and measuring the cable's impedance spectrum. Since the fault point introduces additional impedance, it causes abrupt changes in impedance amplitude and phase at a specific frequency. By comparing this abrupt change with a standard impedance spectrum, the fault point can be located. This application leverages the rich harmonic components of the pulsed square wave excitation signal to effectively obtain the high-frequency impedance characteristics of the shielded cable over a wide frequency range, improving the accuracy and comprehensiveness of the cable's high-frequency characteristic analysis. Furthermore, the use of a bipolar pulsed square wave excitation signal, compared to a unipolar pulsed square wave excitation signal, offers a wider harmonic distribution range and richer high-frequency harmonic components, allowing for impedance spectrum plotting over a broader frequency range after injection into the shielded cable.

[0048] It should be noted that the bipolar pulse square wave excitation signal used in this application is a high-frequency signal, which refers to a signal with a frequency above 1kHz, and specifically, a frequency within the range of 1kHz-100MHz in specific implementations. This frequency range covers the frequency components of high-frequency transient currents commonly found in shielded cables. When acquiring the response signal of the shielded cable, the response signal is also a high-frequency response signal. In step S10, based on the electrical parameters of the shielded cable and the tolerance of the measuring equipment, a bipolar pulse square wave excitation signal with a specific frequency and amplitude is designed and generated to ensure that the injected bipolar pulse square wave excitation signal can effectively excite the shielded cable to exhibit high-frequency characteristics without damaging the measuring equipment or the shielded cable itself. For example, the frequency of the bipolar pulse square wave excitation signal can be set to 10MHz, the rise time to within 20ns, and the amplitude to 50V. This ensures the excitation of the distributed parameter effects of the cable in the high-frequency band, avoids signal distortion caused by strong high-frequency components, and ensures effective excitation of distributed parameter effects such as skin effect and dielectric dispersion characteristics, achieving wide-band excitation without dead angles. After injecting a bipolar pulsed square wave excitation signal into the shielded cable, the response signal of the shielded cable under excitation is collected. The collection points can be set at the input end, output end, critical locations, and sections where defects or aging may exist. The collected response signals are mainly voltage and current signals, and appropriate high-frequency voltage and current sensors can be used for acquisition. High-speed sampling technology can be used during signal acquisition. Based on the frequency of the bipolar pulsed square wave excitation signal, the sampling frequency should be set to no less than 10MHz to ensure accurate acquisition of the high-frequency response signal details of the cable under pulsed square wave excitation. Because the waveform change of the pulsed square wave excitation signal is relatively gradual, it is easier to accurately capture signal characteristics at the same sampling frequency.

[0049] After acquiring the response signal of the shielded cable, impedance calculation can be performed. In step S11, the acquired time-domain response signal needs to be converted into a frequency-domain signal, and the impedance value of the shielded cable at different frequencies is calculated based on the frequency-domain signal. Specifically, this can be achieved by converting the high-frequency voltage and current signals in the time domain into frequency-domain signals using Fourier transform. According to Ohm's law, the impedance value of the cable at different frequencies is calculated in the frequency domain. In practical applications, the impedance-frequency characteristic curve can be plotted based on the changes in the real part (resistance) and imaginary part (reactance) of the impedance value with frequency. This visually demonstrates the changes in the resistance, inductance, and capacitance characteristics of the cable at different high-frequency bands, thereby providing a deeper understanding of the cable's high-frequency electrical performance.

[0050] In step S12, the amplitude-frequency and phase-frequency characteristics are calculated based on the calculated impedance values ​​to obtain the true impedance spectrum. The true impedance spectrum is then compared with the standard impedance spectrum to identify and locate the fault point. It can be understood that in step S13, the fault point is distinguished based on the jumps in the true impedance spectrum, and the fault distance can be calculated based on the wave velocity to locate the fault point.

[0051] This invention provides a cable fault identification method based on bipolar pulsed square wave excitation. Compared to current technologies that use sine waves for cable fault point identification, which suffer from limitations such as single-frequency excitation and difficulty in capturing detailed changes, this application uses a bipolar pulsed square wave excitation signal. Compared to a sine wave, the pulsed square wave excitation signal possesses rich harmonic components, effectively acquiring the high-frequency impedance characteristics of the shielded cable over a wide frequency range, thus improving the accuracy and comprehensiveness of the cable's high-frequency characteristic analysis. Furthermore, compared to a unipolar pulsed square wave excitation signal, the bipolar pulsed square wave excitation signal offers a wider harmonic distribution range and richer high-frequency harmonic components, allowing for impedance spectrum plotting over a broader frequency range after injection into the shielded cable. Using this technical solution, after the bipolar pulsed square wave excitation signal is injected into the shielded cable, the response signal of the shielded cable under excitation is collected to obtain the true impedance spectrum. By comparing and analyzing this spectrum with a standard impedance spectrum generated based on a transmission line model, fault points can be identified and located. This enables cable fault point identification over a wider frequency range and achieves the capture of detailed changes in the cable under excitation.

[0052] It is understandable that the acquisition of impedance spectrum using the frequency sweep method in this application mainly relies on the excitation of bipolar pulse square wave. The bipolar pulse square wave is a periodic non-sinusoidal component that can be expanded into a Fourier series, that is, decomposed into a series of sinusoidal quantities with different frequencies that are superimposed, which includes the fundamental wave and a series of odd harmonics (such as the 3rd, 5th, 7th, etc.). Figure 2 A schematic diagram of a bipolar pulse square wave excitation provided in an embodiment of the present invention is shown below. Figure 2 As shown, the period of the pulsed square wave excitation signal is The amplitude is Then its frequency for:

[0053] ;

[0054] For a bipolar pulse square wave signal, the wavelengths corresponding to each frequency component still satisfy... ( For wavelength, (The fundamental frequency is the speed at which a signal propagates in the transmission medium). Corresponding wavelength , Subharmonic frequency Its corresponding wavelength .

[0055] For an odd-symmetric bipolar pulse square wave, which has no constant term, it can be expressed as follows after Fourier series decomposition:

[0056] ;

[0057] For even-symmetric bipolar pulse square waves, there is no constant term. After decomposition, it is represented as a superposition of cosine functions. Since the sine and cosine differ by 90°, the even-symmetric bipolar pulse square wave lags behind the odd-symmetric bipolar pulse square wave by 90°. Therefore, the decomposed values ​​are the same. To facilitate calculation, the bipolar pulse square wave excitation signal in this application satisfies the odd-symmetric characteristic.

[0058] The above embodiments provide a method for identifying fault points, which involves comparing and analyzing the actual impedance spectrum collected with a standard impedance spectrum, and locating the fault through abrupt changes in amplitude or phase frequency. This application provides a method for obtaining a standard impedance spectrum, which takes into account the skin effect and the complex permittivity of the insulating medium. Following the equations, based on the hardware structure of the shielded cable, the distributed resistance, inductance, capacitance, and conductance are dynamically calculated. This leads to the establishment of a lossy transmission line model with uniformly distributed cable parameters. Based on this lossy transmission line model, and considering the amplitude-frequency / phase-frequency response differences of different harmonics of the bipolar pulse square wave excitation signal propagating in space, the input impedance formula is derived. By utilizing the sensitivity of higher harmonics of the bipolar pulse square wave excitation signal to parameter changes, the distribution differences between amplitude-frequency and phase-frequency characteristics are used to identify parameter anomalies within the cable.

[0059] For the impedance formula, assuming the shielded cable is of length... A uniform transmission line, in The cable input impedance at the location is:

[0060] ;

[0061] In the formula, The load reflection coefficient, For wave impedance, Let be the propagation constant. and All of these depend on the characteristics of the shielded cable itself. By measuring the input impedance frequency response characteristics of the shielded cable, combined with the propagation constant... The position dependence can resolve the exponential decay term in the impedance spectrum. and reflection coefficient The changes in impedance result in differences in the amplitude-frequency and phase-frequency characteristics of the cable impedance spectrum. When there is a localized fracture, corrosion, or poor contact, the sudden change in load impedance leads to… Anomalies are identified by comparing the frequency domain deviations between measured and theoretical impedance spectra to assess cable performance.

[0062] Substituting the function of the bipolar pulse square wave excitation into the cable impedance spectrum calculation, we obtain each harmonic frequency. The input impedance is:

[0063] ,

[0064] in, Given the load impedance, characteristic impedance, and propagation coefficient, they can be expressed as:

[0065] ;

[0066] In the formula, For resistance, For inductance, For capacitors, To determine the conductivity, the impedance spectra at each harmonic frequency are summed to obtain the total impedance spectrum of the cable:

[0067] ;

[0068] in, The fundamental frequency of the bipolar pulse square wave excitation signal is... for Characteristic impedance at the subharmonic frequency. for The propagation coefficient of the subharmonic frequency, The amplitude of the bipolar pulse square wave excitation signal. Indicates frequency at Discrete components at the location, For input impedance, For the length of the shielded cable, For input impedance, It is a constant.

[0069] The impedance spectrum of the shielded cable can be obtained using this formula. When obtaining the standard impedance spectrum, this formula can be used to obtain the standard impedance spectrum based on the established transmission line model.

[0070] As can be understood from the above embodiments, both the excitation signal and the response signal in this application are high-frequency signals. To avoid equipment damage and signal loss when acquiring the response signal, appropriate acquisition equipment is required. Based on the above embodiments, in this embodiment, the voltage signal acquisition device is a voltage sensor with a capacitive voltage divider structure. This structure has wideband response characteristics and can accurately measure high-frequency voltage signals. The current signal acquisition device is a current sensor designed using the Rogowski coil principle. This structure has high sensitivity and linearity for high-frequency currents. The sensors convert the acquired high-frequency voltage and current signals into electrical signals suitable for processing by the measuring equipment and transmit them to the data processing center via shielded cables. This center can be the data acquisition system in a high-frequency impedance analyzer.

[0071] In practical implementation, to compensate for signal attenuation after the excitation signal is injected into the shielded cable and to reduce signal interference, when injecting the bipolar pulse square wave excitation signal into the shielded cable, the bipolar pulse square wave excitation signal is amplified to a preset strength by a power amplifier and then injected into the shielded cable using an isolation transformer. This is also a high-frequency isolation transformer, possessing excellent high-frequency transmission characteristics and electrical isolation performance, which can avoid electrical interference between the measuring equipment and the cable, while ensuring efficient transmission of the bipolar pulse square wave excitation signal.

[0072] After acquiring the response signal, similarly, to reduce interference and improve detection accuracy, following the step of acquiring the response signal of the shielded cable under excitation, the following steps are also included: using a wavelet transform denoising algorithm to remove noise components from the response signal. The wavelet transform denoising algorithm can remove noise components from the acquired signal to the greatest extent while preserving the effective characteristics of the signal. Furthermore, a Butterworth low-pass filter is used to filter the signal, removing high-frequency stray components from the high-frequency response signal, making the signal smoother and facilitating subsequent analysis. To avoid interference from the response signal acquisition equipment and data processing equipment itself, before calculating the amplitude-frequency and phase-frequency characteristics based on the impedance value to obtain the true impedance spectrum, no-load calibration is performed to subtract system noise from the response signal, further ensuring detection accuracy.

[0073] The above embodiments provide a detailed description of the cable fault identification method based on bipolar pulse square wave excitation provided in this application. In order to verify the identification effect of this method, this embodiment is tested through specific experiments.

[0074] In practical implementation, the bipolar pulse square wave signal generator and the high-frequency impedance analyzer are connected at both ends (excitation end and measurement end are on the same line) to inject the excitation signal into the cable and collect the input impedance. Start the bipolar pulse square wave signal generator, with an output base frequency of 10MHz and an amplitude of... A 50V odd-symmetric bipolar pulsed square wave excitation signal is used, with a rise time controlled within 20ns to ensure the excitation of the cable's distributed parameter effects at high frequencies. After power amplification, the signal is directly injected into the cable input. A high-frequency impedance analyzer scans within the synchronous frequency band, acquiring the amplitude-frequency characteristics of the input impedance. Phase frequency characteristics By subtracting system noise through no-load calibration, a true impedance spectrum curve is generated, providing raw data for subsequent analysis. Based on the cable's physical geometric parameters, including cable length, dielectric constant of the insulation material, and shielding structure, the resistance per unit length is calculated using the cable's distributed parameter model. ,inductance ,capacitance and conductivity Complete the parameter configuration of the theoretical model. Generate the impedance spectrum according to the following formula and compare it with the measured impedance spectrum. The maximum harmonic order used in this experiment is 11.

[0075] .

[0076] Figure 3 This is a schematic diagram of a bipolar pulse square wave signal and its Fourier decomposition provided in an embodiment of the present invention. Figure 4 This invention provides a comparison chart of impedance spectrum amplitude characteristics under simulation and actual measurement. Figure 5 This invention provides a comparison diagram of the phase characteristics of impedance spectrum under simulation and actual measurement. As can be seen from the figure, the Fourier series decomposition of the bipolar pulse square wave includes the fundamental wave and odd harmonics, covering the entire high-frequency transient frequency band of the cable system in one go. This solves the problem of the single excitation frequency of traditional sine waves and can comprehensively reflect the changes in the resistance, inductance and capacitance characteristics of the cable over a wide frequency range. Furthermore, the figure shows that the measured values ​​and theoretical values ​​have a high degree of agreement across the entire frequency band.

[0077] The cable fault identification method based on bipolar pulse square wave excitation provided by this invention has been described in detail above. The various embodiments in the specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. It should be noted that those skilled in the art can make various improvements and modifications to this invention without departing from its principles, and these improvements and modifications also fall within the protection scope of the claims of this invention.

Claims

1. A cable fault identification method based on bipolar pulse square wave excitation, characterized in that, include: After a bipolar pulse square wave excitation signal is injected into the shielded cable, the response signal of the shielded cable under excitation is collected. The response signal includes a voltage signal and a current signal; The time-domain response signal is converted into a frequency-domain signal to calculate the impedance value of the shielded cable at different frequencies; The amplitude-frequency and phase-frequency characteristics are calculated based on the impedance values ​​to obtain the true impedance spectrum. The actual impedance spectrum is compared and analyzed with the standard impedance spectrum to identify and locate the fault point.

2. The cable fault identification method based on bipolar pulse square wave excitation according to claim 1, characterized in that, The bipolar pulse square wave excitation signal satisfies odd symmetry characteristics.

3. The cable fault identification method based on bipolar pulse square wave excitation according to claim 2, characterized in that, The formula for obtaining the standard impedance spectrum includes: A lossy transmission line model is constructed based on the hardware structure of the shielded cable; Based on the aforementioned lossy transmission line model, a standard impedance spectrum is generated according to the following formula; ； in, For load impedance, The fundamental frequency of the bipolar pulse square wave excitation signal is... for Characteristic impedance at the subharmonic frequency. for The propagation coefficient of the subharmonic frequency, The amplitude of the bipolar pulse square wave excitation signal. Indicates frequency at Discrete components at the location, For the length of the shielded cable, For input impedance, It is a constant.

4. The cable fault identification method based on bipolar pulse square wave excitation according to claim 1, characterized in that, The voltage signal acquisition device is a voltage sensor with a capacitive voltage divider structure; the current signal acquisition device is a current sensor designed using the Rogowski coil principle.

5. The cable fault identification method based on bipolar pulse square wave excitation according to claim 4, characterized in that, The bipolar pulse square wave excitation signal injection shielded cable includes: The bipolar pulse square wave excitation signal is amplified to a preset intensity by a power amplifier and then injected into the shielded cable using an isolation transformer.

6. The cable fault identification method based on bipolar pulse square wave excitation according to claim 1, characterized in that, Following the step of acquiring the response signal of the shielded cable under excitation, the method further includes: The wavelet transform denoising algorithm is used to remove noise components from the response signal.

7. The cable fault identification method based on bipolar pulse square wave excitation according to claim 6, characterized in that, Also includes: The response signal is filtered using a Butterworth low-pass filter.

8. The cable fault identification method based on bipolar pulse square wave excitation according to claim 7, characterized in that, Before the step of calculating the amplitude-frequency and phase-frequency characteristics based on the impedance values ​​to obtain the true impedance spectrum, the method further includes: The system noise in the response signal is subtracted by no-load calibration.

9. The cable fault identification method based on bipolar pulse square wave excitation according to claim 1, characterized in that, Also includes: Based on the variation of the real and imaginary parts of the impedance value with frequency, impedance frequency characteristic curves are plotted to analyze the resistance, inductance, and capacitance variation characteristics of the shielded cable in different frequency bands.

10. The cable fault identification method based on bipolar pulse square wave excitation according to any one of claims 1 to 9, characterized in that, The frequency of the bipolar pulse square wave excitation signal is 10MHz, and the rise time is within 20ns.

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