Cable high-frequency impedance characteristic analysis method based on bipolar trapezoidal wave excitation

Abstract

The invention discloses a cable high-frequency impedance characteristic analysis method based on bipolar trapezoidal wave excitation, and belongs to the technical field of power equipment state monitoring and fault diagnosis. In order to solve the problems of low test efficiency, insufficient high-frequency parameter modeling precision and weak anti-interference capability in cable high-frequency impedance characteristic analysis of a traditional sine wave excitation method, the method comprises the following steps: generating an odd-symmetry bipolar trapezoidal wave excitation signal with specific frequency and amplitude, and injecting the signal into the head end of a cable to be tested through a coupling device after power amplification; synchronously acquiring a high-frequency response signal of the key position of the cable under the excitation of the bipolar trapezoidal wave by using a data acquisition system; preprocessing the high-frequency response signal, converting the high-frequency response signal into a frequency domain through Fourier transform, calculating input impedance of the cable under different harmonic frequencies, and synthesizing a broadband impedance spectrum; based on the broadband impedance spectrum, the amplitude-frequency characteristic and the phase-frequency characteristic of impedance are analyzed, and cable state evaluation, partial discharge detection, insulation defect diagnosis or fault point positioning are realized. The method is used for an electric power system.

Description

Technical Field

[0001] This invention relates to a method for analyzing the high-frequency impedance characteristics of cables based on bipolar trapezoidal wave excitation, belonging to the field of power equipment condition monitoring and fault diagnosis technology. Background Technology

[0002] As a core component for the safe and reliable transmission and distribution of electrical energy, the stable operation of the power system directly affects the normal functioning of social production and daily life. In the power transmission and distribution network, cables bear the critical task of transmitting electrical energy and signals. Cross-linked polyethylene (XLPE) cables, especially those in the 10–220 kV voltage range, are widely used in urban power grids due to their excellent dielectric and physicochemical properties. However, XLPE cables are typically designed for a lifespan of 20–30 years. In actual operation, they are often laid in complex environments such as cable trenches, pipes, walls, or direct underground burial, and are subject to long-term exposure to factors such as temperature, humidity, mechanical stress, and chemical corrosion, making them prone to insulation defects. These defects accelerate the degradation of cable insulation performance, shorten their service life, and consequently threaten the overall reliability of the power system.

[0003] To address the aforementioned issues, scholars both domestically and internationally have proposed various technologies for detecting cable insulation defects and faults. Traditional reflected wave methods and their derivatives (such as broadband impedance spectroscopy) inject sinusoidal sweep signals into the cable, measure the input impedance point-by-point, and construct an impedance spectrum to identify fault characteristics. However, these methods have the following inherent limitations:

[0004] 1. Low testing efficiency and complex system: Sine sweep frequency requires point-by-point excitation and data splicing, which is time-consuming and difficult to meet the needs of rapid on-site testing; at the same time, it relies on high-precision signal generators and synchronous acquisition equipment, resulting in high system cost and complex deployment and maintenance.

[0005] 2. Insufficient accuracy in high-frequency parameter modeling: Existing equivalent models often ignore the skin effect, proximity effect and dispersion characteristics of insulating materials at high frequencies, simplifying parameters to constants, resulting in serious deviations between high-frequency simulations and actual responses.

[0006] 3. Insufficient fault feature extraction: Relying on pure frequency domain spectrum matching, it fails to make full use of the time domain location information in transient traveling waves, resulting in insufficient sensitivity to early faults and minor defects.

[0007] In recent years, some studies have attempted to improve measurement accuracy through dual-path coupling frequency domain reflection technology (such as CN120507604A), but it is still based on sinusoidal frequency sweep mechanism, which has problems such as energy dispersion, low high-frequency signal-to-noise ratio and weak anti-interference ability. Moreover, the synchronization of multiple signals and complex data processing algorithms further increase the system complexity and real-time challenges.

[0008] Furthermore, existing methods generally assume that cable distributed parameters have no frequency-dependent effects, neglecting the significant changes in conductor resistance and inductance at high frequencies, leading to biases in impedance characteristic analysis. Therefore, there is an urgent need for a highly efficient detection method that can comprehensively cover a wide frequency range, accurately capture high-frequency parameter changes, and possess strong anti-interference capabilities, in order to improve the accuracy and reliability of cable condition assessment and fault diagnosis. Summary of the Invention

[0009] The purpose of this invention is to solve the problems of low testing efficiency, insufficient accuracy of high-frequency parameter modeling, and weak anti-interference ability of traditional sinusoidal wave excitation methods in the analysis of high-frequency impedance characteristics of cables, and to provide a method for analyzing high-frequency impedance characteristics of cables based on bipolar trapezoidal wave excitation.

[0010] The present invention discloses a method for analyzing the high-frequency impedance characteristics of cables based on bipolar trapezoidal wave excitation, comprising:

[0011] In the signal injection step, an odd-symmetric bipolar trapezoidal wave excitation signal with a specific frequency and amplitude is generated, amplified by power, and then injected into the beginning of the cable under test through a coupling device.

[0012] The signal acquisition step involves using a data acquisition system to synchronously acquire the high-frequency response signals at key locations of the cable under bipolar trapezoidal wave excitation.

[0013] The signal processing and impedance calculation steps involve preprocessing the acquired high-frequency response signal, converting it to the frequency domain via Fourier transform, calculating the input impedance of the cable at different harmonic frequencies, and synthesizing a broadband impedance spectrum.

[0014] The characteristic analysis and diagnostic steps, based on the broadband impedance spectrum, analyze the amplitude-frequency and phase-frequency characteristics of the impedance to achieve cable condition assessment, partial discharge detection, insulation defect diagnosis, or fault location.

[0015] Preferably, the fundamental frequency range of the bipolar trapezoidal wave excitation signal is 1kHz to 120MHz, the amplitude is 50V, the rise time and fall time are independently controllable, and are less than or equal to 20 to 2000ns.

[0016] Preferably, the high-frequency response signal includes a high-frequency voltage response signal and a high-frequency current response signal.

[0017] Preferably, the high-frequency voltage response signal is acquired using a high-frequency voltage sensor; the high-frequency voltage sensor is a capacitive voltage divider structure and is configured to achieve distortion-free measurement of the high-frequency voltage signal within the wide bandwidth of the bipolar trapezoidal wave excitation signal;

[0018] The high-frequency current response signal is acquired using a high-frequency current sensor, which is constructed based on the Rogowski coil principle and configured to maintain high sensitivity and linearity in response to the high-frequency current signal.

[0019] Preferably, the key locations of the cable include: cable joints, cable terminals, and sections that may have defects or be aged.

[0020] Preferably, the coupling device uses a high-frequency isolation transformer, with the primary side connected to the output terminal of the power amplifier and the secondary side connected in series between the first conductor of the cable under test and the grounding terminal;

[0021] The high-frequency isolation transformer is configured to have a flat amplitude frequency response and a linear phase frequency response within the fundamental and harmonic frequency bands of the bipolar trapezoidal wave excitation signal.

[0022] Preferably, in the signal injection step, a two-terminal common-line connection is adopted, in which the bipolar trapezoidal wave signal generator and the excitation and measurement ends of the high-frequency impedance analyzer are connected to the beginning of the cable; before injecting the excitation signal, an unloaded calibration procedure is performed to deduct the inherent noise of the system and obtain the true cable input impedance.

[0023] Preferably, the characteristic analysis and diagnosis step further includes a theoretical model construction step:

[0024] Based on the physical geometric parameters of the cable, a distributed parameter model is established using the uniform transmission line theory.

[0025] The resistance per unit length as a function of frequency was calculated. ,inductance ,capacitance and conductivity And based on this, a theoretical input impedance spectrum is generated;

[0026] In the distributed parameter model, the complex dielectric constant of the insulating material is calculated based on the Cole-Cole equation and in combination with the actual cable type and material parameters.

[0027] Preferably, in the characteristic analysis and diagnosis step, the measured broadband impedance spectrum and the theoretical input impedance spectrum are compared in terms of amplitude frequency characteristics and phase frequency characteristics within a frequency range of 1MHz to 110MHz. By identifying the deviation characteristics between the two at a specific frequency, the cable insulation condition assessment and fault location are realized.

[0028] Preferably, the broadband impedance spectrum is calculated according to the following transmission line model formula:

[0029]

[0030] in, This represents the total impedance spectrum of the cable. The frequency representing the impedance spectrum is the independent variable of the impedance function. This represents the amplitude of the odd-symmetric bipolar trapezoidal wave excitation signal. Indicates the order of harmonics. , The parameter representing the duty cycle of the trapezoidal wave. Indicates the fundamental frequency.

[0031] Indicates the characteristic impedance of the cable. Indicates the cable load impedance. Represents the propagation constant. Indicates the cable length. Represents the Dirac function, Indicates frequency at Discrete components at a given location.

[0032] The present invention proposes a method for analyzing the high-frequency impedance characteristics of cables based on bipolar trapezoidal wave excitation, which has the following significant advantages:

[0033] 1. This invention achieves a leap from "point-by-point scanning" to "parallel analysis," significantly improving testing efficiency: Utilizing the inherent rich odd harmonic components of bipolar trapezoidal wave signals as a natural parallel test signal source, a single signal injection can excite the cable's comprehensive response at the fundamental frequency and multiple harmonic frequencies, equivalent to simultaneously testing dozens or even hundreds of frequency points. This fundamentally avoids the cumbersome frequency switching and data stitching process, reducing testing time by several times and greatly improving detection efficiency, better meeting the urgent need for rapid on-site diagnosis. Simultaneously, this method reduces reliance on high-precision, expensive sweep signal generation and synchronous acquisition systems, simplifying the system architecture and lowering hardware costs and on-site deployment and maintenance difficulties.

[0034] 2. Improved accuracy of high-frequency impedance characteristic analysis, enabling precise capture of frequency-varying effects: The bipolar trapezoidal wave used in this invention has controllable, extremely steep rise / fall times (up to 20ns), and its high-frequency components can effectively excite the distributed parameter response of the cable in the high-frequency band. Combined with the established precise distributed parameter model considering the skin effect, proximity effect, and dielectric dispersion of the insulating material, it can accurately characterize frequency-varying phenomena such as increased high-frequency resistance and decreased inductance of the conductor. Experiments show that the impedance calculation error of this method in the high-frequency band (such as above 10MHz) can be reduced by more than 70% compared with the traditional simplified model, providing a more reliable data foundation for fault diagnosis.

[0035] 3. Improved sensitivity and accuracy of fault diagnosis and location, combining the advantages of both time and frequency domains: This invention innovatively integrates the time-domain reflection principle with frequency-domain impedance analysis. On the one hand, the clear waveform edges of the bipolar trapezoidal wave propagating in the cable generate reflected waves when they encounter a fault point. By analyzing the time difference between the incident and reflected waves, high-precision preliminary location at the meter or even sub-meter level can be achieved in the time domain. On the other hand, frequency-domain deviation analysis based on broadband impedance spectrum and theoretical models can sensitively capture minute and continuous changes in distributed parameters caused by insulation aging, partial discharge, etc., enabling accurate identification and location of early defects and distributed faults. This strategy of "coarse location in the time domain + fine diagnosis in the frequency domain" overcomes the limitations of single-domain analysis and significantly improves the comprehensive diagnostic capability for complex faults. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of bipolar trapezoidal wave excitation;

[0037] Figure 2 It is a bipolar trapezoidal waveform signal diagram;

[0038] Figure 3 This is a comparison chart of impedance spectrum amplitude characteristics;

[0039] Figure 4 This is a comparison diagram of the phase characteristics of the impedance spectrum. Detailed Implementation

[0040] 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 skilled in the art without creative effort are within the scope of protection of the present invention.

[0041] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0042] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but this is not intended to limit the scope of the invention.

[0043] Example 1:

[0044] The following is combined Figures 1-4 This embodiment describes a method for analyzing the high-frequency impedance characteristics of cables based on bipolar trapezoidal wave excitation, which includes:

[0045] In the signal injection step, an odd-symmetric bipolar trapezoidal wave excitation signal with a specific frequency and amplitude is generated, amplified by power, and then injected into the beginning of the cable under test through a coupling device.

[0046] The signal acquisition step involves using a data acquisition system to synchronously acquire the high-frequency response signals at key locations of the cable under bipolar trapezoidal wave excitation.

[0047] The signal processing and impedance calculation steps involve preprocessing the acquired high-frequency response signal, converting it to the frequency domain via Fourier transform, calculating the input impedance of the cable at different harmonic frequencies, and synthesizing a broadband impedance spectrum.

[0048] The characteristic analysis and diagnostic steps, based on the broadband impedance spectrum, analyze the amplitude-frequency and phase-frequency characteristics of the impedance to achieve cable condition assessment, partial discharge detection, insulation defect diagnosis, or fault location.

[0049] Furthermore, the fundamental frequency range of the bipolar trapezoidal wave excitation signal is 1kHz to 120MHz, the amplitude is 50V, and the rise and fall times are independently controllable and less than or equal to 20 to 2000ns.

[0050] Furthermore, the high-frequency response signal includes a high-frequency voltage response signal and a high-frequency current response signal.

[0051] Furthermore, the high-frequency voltage response signal is acquired using a high-frequency voltage sensor; the high-frequency voltage sensor is a capacitive voltage divider structure and is configured to achieve distortion-free measurement of the high-frequency voltage signal within the wide bandwidth of the bipolar trapezoidal wave excitation signal.

[0052] The high-frequency current response signal is acquired using a high-frequency current sensor, which is constructed based on the Rogowski coil principle and configured to maintain high sensitivity and linearity in response to the high-frequency current signal.

[0053] Furthermore, the key locations of the cable include: cable joints, cable terminals, and sections that may have defects or be aged.

[0054] Furthermore, the coupling device employs a high-frequency isolation transformer, with its primary side connected to the output terminal of the power amplifier and its secondary side connected in series between the first conductor of the cable under test and the grounding terminal.

[0055] The high-frequency isolation transformer is configured to have a flat amplitude frequency response and a linear phase frequency response within the fundamental and harmonic frequency bands of the bipolar trapezoidal wave excitation signal.

[0056] Furthermore, in the signal injection step, a two-terminal common-line connection is adopted, connecting the bipolar trapezoidal wave signal generator and the excitation and measurement ends of the high-frequency impedance analyzer together at the beginning of the cable; before injecting the excitation signal, an unloaded calibration procedure is performed to deduct the inherent noise of the system and obtain the true cable input impedance.

[0057] Furthermore, the aforementioned characteristic analysis and diagnostic steps also include a theoretical model construction step:

[0058] Based on the physical geometric parameters of the cable, a distributed parameter model is established using the uniform transmission line theory.

[0059] The resistance per unit length as a function of frequency was calculated. ,inductance ,capacitance and conductivity And based on this, a theoretical input impedance spectrum is generated;

[0060] In the distributed parameter model, the complex dielectric constant of the insulating material is calculated based on the Cole-Cole equation and in combination with the actual cable type and material parameters.

[0061] Furthermore, in the characteristic analysis and diagnosis step, the measured broadband impedance spectrum and the theoretical input impedance spectrum are compared in terms of amplitude-frequency characteristics and phase-frequency characteristics within the frequency range of 1MHz to 110MHz. By identifying the deviation characteristics between the two at specific frequencies, the cable insulation condition is assessed and fault location is determined.

[0062] Furthermore, the broadband impedance spectrum is calculated using the following transmission line model formula:

[0063]

[0064] in, This represents the total impedance spectrum of the cable. The frequency representing the impedance spectrum is the independent variable of the impedance function. This represents the amplitude of the odd-symmetric bipolar trapezoidal wave excitation signal. Indicates the order of harmonics. , The parameter representing the duty cycle of the trapezoidal wave. Indicates the fundamental frequency.

[0065] Indicates the characteristic impedance of the cable. Indicates the cable load impedance. Represents the propagation constant. Indicates the cable length. Represents the Dirac function, Indicates frequency at Discrete components at a given location.

[0066] In order to overcome the shortcomings of the existing technology, this invention provides a method and application for analyzing the high-frequency impedance characteristics of cables based on bipolar trapezoidal wave excitation. By using a bipolar trapezoidal wave excitation signal and making full use of the rich harmonic components of the bipolar trapezoidal wave, the high-frequency impedance characteristics of the cable in a wide frequency range can be effectively obtained. This overcomes the shortcomings of the traditional sinusoidal wave excitation method, improves the accuracy and comprehensiveness of the high-frequency characteristic analysis of the cable, and provides a reliable basis for cable condition assessment and fault diagnosis.

[0067] Characteristics and decomposition of bipolar trapezoidal wave excitation signal: Bipolar trapezoidal 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 superimposed, which includes the fundamental wave and a series of odd harmonics (such as the 3rd, 5th, 7th, etc.).

[0068] Based on period Amplitude Bipolar trapezoidal waves, such as Figure 1 As shown, that is:

[0069]

[0070] In the formula: For the rising time, For the time of the flat top, .

[0071] For a bipolar trapezoidal wave signal, the wavelengths corresponding to each frequency component still satisfy... ( The speed at which a signal propagates in the transmission medium. (Frequency), i.e., fundamental frequency Corresponding wavelength , Subharmonic frequency (n=1,3,5,...,2n-1), corresponding wavelength .

[0072] For an odd-symmetric bipolar trapezoidal wave, after Fourier series decomposition, it can be expressed as:

[0073]

[0074] In the formula: For bipolar trapezoidal wave amplitude, The fundamental frequency of the bipolar trapezoidal wave ( , (for periodicity) The proportion of the rise time of the trapezoidal wave to half a cycle. .

[0075] For an odd-symmetric bipolar trapezoidal wave, after decomposition, it can be represented as a superposition of sine functions, exhibiting clear odd harmonic characteristics and having no DC component. Compared to even-symmetric waveforms, odd-symmetric trapezoidal waves avoid the influence of DC bias, simplify harmonic impedance matching calculations, are more conducive to the transmission efficiency of high-frequency signals, and are more consistent with practical systems. This invention uses odd-symmetric bipolar trapezoidal waves.

[0076] Cable impedance spectrum analysis:

[0077] Basic assumption: The cable is of length... A uniform transmission line.

[0078] exist The cable input impedance at the location is:

[0079]

[0080] 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 cable itself. By measuring the cable's input impedance frequency response characteristics, 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.

[0081] Substituting the function of the bipolar trapezoidal wave excitation into the cable impedance spectrum calculation, we obtain the frequency of each harmonic. The input impedance is:

[0082]

[0083] in, Given the load impedance, characteristic impedance, and propagation coefficient, they can be expressed as:

[0084]

[0085] The impedance spectra at each harmonic frequency are summed to obtain the total impedance spectrum of the cable:

[0086]

[0087]

[0088] In the formula, Indicates frequency at Discrete components at that point, Given the zero-frequency input impedance, the cable impedance spectrum obtained from equation (6) can be used to evaluate the high-frequency performance of the cable.

[0089] The technical solution of the present invention is as follows:

[0090] Bipolar trapezoidal wave excitation signal injection:

[0091] (1) Design and generate a bipolar trapezoidal wave excitation signal with a specific frequency and amplitude. The trapezoidal wave frequency range is set to 1kHz-1MHz, which covers the frequency components of common high-frequency transient currents in cables. The bipolar trapezoidal wave has a controllable rising edge, flat-top segment, and falling edge. The high-frequency harmonic distribution can be controlled by adjusting the flat-top time, which is more conducive to specifically exciting the response characteristics of the cable in different frequency bands. The signal amplitude is reasonably adjusted according to the electrical parameters of the cable and the tolerance of the measuring equipment to ensure that the injected rectangular wave signal can effectively excite the cable to exhibit high-frequency characteristics without damaging the measuring equipment and the cable itself.

[0092] (2) The generated bipolar trapezoidal wave excitation signal is amplified to a sufficient intensity by a power amplifier, and then the amplified bipolar trapezoidal wave signal is safely and stably injected into the cable using a coupling device. The coupling device uses a specially designed high-frequency isolation transformer, which has good 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 trapezoidal wave signal.

[0093] High-frequency response signal acquisition:

[0094] (1) High-frequency voltage sensors and high-frequency current sensors are placed at key locations such as the input and output ends of the cable. The location of the sensors is optimized according to the structural characteristics of the cable and the analysis requirements. For example, sensors are placed at cable joints, terminals, and sections where defects and aging may exist, in order to fully capture the high-frequency response signal of the cable under bipolar trapezoidal wave excitation.

[0095] (2) The high-frequency voltage sensor adopts a capacitive voltage divider structure, which has wide-band response characteristics and can accurately measure high-frequency voltage signals. The high-frequency current sensor adopts the Rogowski coil principle, which has high sensitivity and linearity for high-frequency current. The sensor converts the collected high-frequency voltage and current signals into electrical signals suitable for processing by the measuring equipment, and transmits them to the data acquisition system through a shielded cable.

[0096] (3) The data acquisition system adopts high-speed sampling technology with a sampling frequency of no less than 10MHz to ensure accurate acquisition of the high-frequency response signal details of the cable under bipolar trapezoidal wave excitation. The clear waveform characteristics of the trapezoidal wave (such as steep edges and stable flat tops) make it easier to identify in the time domain, which helps to perform accurate waveform alignment and feature extraction in post-processing, thereby reducing analysis errors. At the same time, the data acquisition system has anti-interference function, which effectively suppresses the influence of external high-frequency interference signals on the acquired data through a combination of hardware filtering and software algorithms.

[0097] High-frequency impedance characteristic calculation:

[0098] (1) Preprocessing of the high-frequency response signal of the cable includes noise removal, filtering, and signal calibration. Wavelet transform denoising algorithm is used to remove noise components in the acquired signal to the greatest extent while preserving the effective features of the signal. Butterworth low-pass filter is used to filter the signal to remove high-frequency spurious components in the high-frequency response signal, making the signal smoother and facilitating subsequent analysis.

[0099] (2) The preprocessed time-domain high-frequency voltage and current signals are converted into frequency-domain signals using Fourier transform. Thanks to the more concentrated and controllable harmonic components of bipolar trapezoidal waves, impedance spectrum data with a higher signal-to-noise ratio can be obtained from the frequency-domain response. Based on Ohm's law, the impedance values ​​of the cable at different frequencies are calculated in the frequency domain.

[0100] (3) Further analyze the variation of the real part (resistance) and imaginary part (reactance) of the cable impedance with frequency. By plotting the impedance frequency characteristic curve, the changes in the resistance, inductance and capacitance characteristics of the cable in the high-frequency range can be intuitively displayed, thereby gaining a deeper understanding of the high-frequency electrical performance of the cable.

[0101] The method of this invention and the high-frequency impedance characteristic analysis results of cables obtained based on it can be widely applied to various aspects of power cable systems, such as cable condition assessment, partial discharge detection, insulation aging diagnosis, and fault location. It can provide targeted solutions for the safe and stable operation of cable systems, has significant practical application value, and helps improve the overall reliability and safety of power systems.

[0102] This invention proposes a method and application for analyzing the high-frequency impedance characteristics of cables based on bipolar trapezoidal wave excitation, comprising the following steps:

[0103] step The bipolar trapezoidal wave signal generator and the high-frequency impedance analyzer are connected at both ends (excitation and measurement ends are on the same line). The excitation signal is injected into the cable and the input impedance is collected. .

[0104] step : Start the trapezoidal wave signal generator, outputting a fundamental frequency of 10MHz and an amplitude of A 50V odd-symmetric bipolar trapezoidal wave 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.

[0105] step Based on the physical and geometric parameters of the cable, including cable length, dielectric constant of 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.

[0106] step According to the formula

[0107]

[0108] Compare and analyze the generated impedance spectrum with the measured impedance spectrum.

[0109] In this invention, in step In this context, the cable is considered a uniform transmission line, and its distributed parameters remain consistent along its length.

[0110] In this invention, in step In this context, the complex permittivity of insulating materials is based on... The equations were calculated based on the actual cable type and material parameters.

[0111] In this invention, in step In the above, the maximum harmonic order obtained by performing a wideband sweep from 1MHz to 110MHz on a bipolar trapezoidal wave with a fundamental frequency of 10MHz is: .

[0112] This invention significantly improves the accuracy and comprehensiveness of high-frequency characteristic analysis of cables through bipolar trapezoidal wave excitation and impedance spectrum analysis.

[0113] 1. Wideband coverage: Bipolar trapezoidal wave (e.g.) Figure 2The waveform shown consists of a rising edge, a flat-top segment, and a falling edge. The harmonic distribution generated by its Fourier decomposition can be controlled by adjusting the duty cycle. Compared to traditional excitation signals such as square waves (slow harmonic attenuation, excessive high-frequency energy) or triangular waves (fixed harmonic attenuation, single excitation mode), the trapezoidal wave used in this invention can actively and controllably concentrate the main harmonic energy in a specific frequency band. This flexible excitation strategy ensures high-intensity, high signal-to-noise ratio focused detection of the cable's key characteristic frequency bands (such as resonant points and fault-sensitive frequency bands) within a wide frequency band of 1kHz-110MHz, thereby comprehensively and profoundly revealing the cable's broadband impedance characteristics with efficiency far exceeding that of traditional sine wave frequency sweeping.

[0114] 2. Precise High-Frequency Parameter Capture: The unique waveform structure of the bipolar trapezoidal wave enables it to simultaneously excite both transient and steady-state responses of the cable. Steep rising / falling edges primarily excite high-frequency transient characteristics, facilitating the precise capture of phenomena such as increased effective conductor resistance at high frequencies; while the flat top phase facilitates the observation of steady-state conduction characteristics in the mid-to-low frequency range. This segmented excitation characteristic makes it more sensitive to parameter variations across the entire frequency band, and because the excitation intensity is controllable, it avoids nonlinear distortion of local cable parameters caused by excessively strong high-frequency impacts, thereby significantly reducing errors in high-frequency impedance calculations.

[0115] 3. Enhanced Anti-interference Capability: The bipolar trapezoidal wave exhibits clear and regular waveform characteristics, making it easy to identify and accurately align in the time domain. This feature, combined with high-frequency isolation transformer hardware isolation, wavelet transform software denoising, multi-harmonic redundancy verification (such as impedance consistency analysis at different harmonic frequencies), and impedance spectrum comparison, effectively suppresses environmental noise and measurement errors. Its periodic bipolar structure also helps to further eliminate DC bias and low-frequency noise through superposition averaging techniques, thereby comprehensively improving the signal-to-noise ratio and measurement reliability at both the source and post-processing stages.

[0116] 4. Axial Distribution Characteristic Analysis and Precise Fault Location: By analyzing the impedance distribution differences between the cable's input and output ends (such as spatial variations in amplitude-frequency and phase-frequency characteristics), abnormal distribution parameters caused by insulation aging, partial discharge, joint oxidation, or mechanical damage can be identified. Bipolar trapezoidal waves can specifically provide stronger excitation energy in the fault-sensitive frequency band, significantly amplifying the impedance characteristic differences between the fault point and the normal area in the corresponding frequency band. Compared to the smooth characteristics under sinusoidal excitation, the fault characteristic differences under trapezoidal wave excitation are more significant, greatly reducing the difficulty of fault point identification (e.g., Figure 3 and Figure 4 As shown in the figure, by combining multi-harmonic feature comparison and deviation analysis, the accuracy of fault location can be significantly improved, providing accurate frequency domain deviation basis for defect location, and solving the problem that traditional methods are difficult to locate minor or distributed defects in cables.

[0117] This invention utilizes the flexible and controllable harmonic components of bipolar trapezoidal waves. By adjusting its duty cycle and edge rate, the harmonic energy is focused on the key frequency band for cable fault diagnosis, achieving targeted excitation of the cable's high-frequency characteristics. The frequency range covers 1kHz-110MHz, and the rise / fall time can be independently and precisely controlled. It can effectively excite the high-frequency distributed parameter effect using steep edges, and examine the mid-to-low frequency steady-state characteristics through the flat-top section. It can avoid signal distortion caused by strong high-frequency components and achieve wide-band excitation without dead angles.

[0118] A lossy transmission line model with uniform cable distribution parameters is established. The input impedance formula is derived by combining the amplitude-frequency / phase-frequency response differences of different harmonics of bipolar trapezoidal waves propagating in space. The harmonic energy is actively controlled within a specific frequency band by utilizing the flexible and controllable characteristics of the harmonic components of bipolar trapezoidal waves. The abnormal parameters in the cable are identified by the distribution differences of amplitude-frequency characteristics and phase-frequency characteristics.

[0119] High-frequency distributed resistance calculation takes into account a variety of complex factors, enabling frequency-dependent calculations of distributed resistance, inductance, capacitance, and conductance.

[0120] Employing a dual-end collinear excitation and measurement architecture, the bipolar trapezoidal wave combines the advantages of low electromagnetic radiation and fast response due to its controllable edge characteristics, coupled with hardware isolation from a high-frequency isolation transformer. By subtracting system noise through no-load calibration, extracting the harmonic components of the bipolar trapezoidal wave using Fourier transform, and superimposing a wavelet denoising algorithm, the system leverages the concentrated and stable distribution of harmonic energy in the bipolar trapezoidal wave to achieve multi-harmonic redundancy verification (impedance deviation of different harmonics ≤3%), significantly improving anti-interference capability compared to traditional excitation methods.

[0121] Based on the frequency domain deviation between the measured impedance spectrum and the theoretical model, the aging of cable insulation, the location and type of defects can be diagnosed, and fault early warning and location can be achieved.

[0122] This invention overcomes the limitation of traditional sinusoidal excitation methods with a single frequency, enabling effective analysis of high-frequency impedance characteristics of cables over a wide frequency range. It accurately captures cable parameter variations caused by factors such as the skin effect and the dispersion characteristics of insulation materials in the high-frequency band, improving the accuracy of high-frequency characteristic analysis. It enhances anti-interference capabilities, providing a reliable basis for high-frequency performance evaluation of cables. Furthermore, it establishes a cable input impedance spectrum calculation model, enabling cable fault location based on high-frequency impedance characteristics.

[0123] While the invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that different dependent claims and features described herein can be combined in ways different from those described in the original claims. It is also understood that features described in conjunction with individual embodiments can be used in other described embodiments.

Claims

1. A method for analyzing the high-frequency impedance characteristics of cables based on bipolar trapezoidal wave excitation, characterized in that, It includes: In the signal injection step, an odd-symmetric bipolar trapezoidal wave excitation signal with a specific frequency and amplitude is generated, amplified by power, and then injected into the beginning of the cable under test through a coupling device. The signal acquisition step involves using a data acquisition system to synchronously acquire the high-frequency response signals at key locations of the cable under bipolar trapezoidal wave excitation. The signal processing and impedance calculation steps involve preprocessing the acquired high-frequency response signal, converting it to the frequency domain via Fourier transform, calculating the input impedance of the cable at different harmonic frequencies, and synthesizing a broadband impedance spectrum. The characteristic analysis and diagnostic steps, based on the broadband impedance spectrum, analyze the amplitude-frequency and phase-frequency characteristics of the impedance to achieve cable condition assessment, partial discharge detection, insulation defect diagnosis, or fault location.

2. The method for analyzing the high-frequency impedance characteristics of cables based on bipolar trapezoidal wave excitation according to claim 1, characterized in that, The fundamental frequency range of the bipolar trapezoidal wave excitation signal is 1kHz to 120MHz, the amplitude is 50V, and the rise and fall times are independently controllable and less than or equal to 20 to 2000ns.

3. The method for analyzing the high-frequency impedance characteristics of cables based on bipolar trapezoidal wave excitation according to claim 1, characterized in that, The high-frequency response signal includes a high-frequency voltage response signal and a high-frequency current response signal.

4. The method for analyzing the high-frequency impedance characteristics of cables based on bipolar trapezoidal wave excitation according to claim 3, characterized in that, The high-frequency voltage response signal is acquired using a high-frequency voltage sensor; the high-frequency voltage sensor is a capacitive voltage divider structure and is configured to achieve distortion-free measurement of the high-frequency voltage signal within the wide bandwidth of the bipolar trapezoidal wave excitation signal; The high-frequency current response signal is acquired using a high-frequency current sensor, which is constructed based on the Rogowski coil principle and configured to maintain high sensitivity and linearity in response to the high-frequency current signal.

5. The method for analyzing the high-frequency impedance characteristics of cables based on bipolar trapezoidal wave excitation according to claim 1, characterized in that, The key locations of the cable include: cable joints, cable terminals, and sections that may have defects or be aged.

6. The method for analyzing the high-frequency impedance characteristics of cables based on bipolar trapezoidal wave excitation according to claim 1, characterized in that, The coupling device uses a high-frequency isolation transformer, with the primary side connected to the output terminal of the power amplifier and the secondary side connected in series between the first conductor of the cable under test and the grounding terminal. The high-frequency isolation transformer is configured to have a flat amplitude frequency response and a linear phase frequency response within the fundamental and harmonic frequency bands of the bipolar trapezoidal wave excitation signal.

7. The method for analyzing the high-frequency impedance characteristics of cables based on bipolar trapezoidal wave excitation according to claim 1, characterized in that, In the signal injection step, a two-terminal common-line connection is adopted, and the excitation end and measurement end of the bipolar trapezoidal wave signal generator and the high-frequency impedance analyzer are connected to the beginning of the cable. Before injecting the excitation signal, an unloaded calibration procedure is performed to deduct the inherent noise of the system and obtain the true cable input impedance.

8. The method for analyzing the high-frequency impedance characteristics of cables based on bipolar trapezoidal wave excitation according to claim 1, characterized in that, The characteristic analysis and diagnosis steps also include a theoretical model construction step: Based on the physical geometric parameters of the cable, a distributed parameter model is established using the uniform transmission line theory. The resistance per unit length as a function of frequency was calculated. ,inductance ,capacitance and conductivity And based on this, a theoretical input impedance spectrum is generated; In the distributed parameter model, the complex dielectric constant of the insulating material is calculated based on the Cole-Cole equation and in combination with the actual cable type and material parameters.

9. The method for analyzing the high-frequency impedance characteristics of cables based on bipolar trapezoidal wave excitation according to claim 8, characterized in that, In the characteristic analysis and diagnosis steps, the measured broadband impedance spectrum and the theoretical input impedance spectrum are compared in terms of amplitude frequency characteristics and phase frequency characteristics within the frequency range of 1MHz to 110MHz. By identifying the deviation characteristics between the two at specific frequencies, the cable insulation condition is assessed and fault location is determined.

10. The method for analyzing the high-frequency impedance characteristics of cables based on bipolar trapezoidal wave excitation according to claim 1, characterized in that, The broadband impedance spectrum is calculated according to the following transmission line model formula: in, This represents the total impedance spectrum of the cable. The frequency representing the impedance spectrum is the independent variable of the impedance function. This represents the amplitude of the odd-symmetric bipolar trapezoidal wave excitation signal. Indicates the order of harmonics. , The parameter representing the duty cycle of the trapezoidal wave. Indicates the fundamental frequency. Indicates the characteristic impedance of the cable. Indicates the cable load impedance. Represents the propagation constant. Indicates the cable length. Represents the Dirac function, Indicates frequency at Discrete components at a given location.

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